Friend on the Force

I wished I was one of those private eyes in books; they've all got a friend on the force.

—Mask Market

A Friend on the Force is a regular police officer whom a Private Detective or Amateur Sleuth is friendly with that provides assistance to the Private Detective. The Friend on the Force often acts as the official with the authority to arrest the criminals that the Private Detective identifies. Compare to Inspector Lestrade, which is a detective that is incapable of solving crimes without the help a Great Detective.

A supertrope for Cop Boyfriend.

Examples of Friend on the Force include:

Anime and Manga

The Big O's Roger Smith has Dan Dastun.
Jet Black from Cowboy Bebop still has friends inside the ISSP.
Rally Vincent has Roy Coleman.
Friend of a Magical Girl rather than a private detective: In Codename: Sailor V, Natsuna Sakurada, Superindent-General of the Metropolitan Police Board, knows (at the end of the series) that Minako Aino is Sailor V and is in regular contact with her.

Comic Books

Ms. Tree has Lt. Rafe Valer, her late husband's former partner.
Lt. Roberta Bliss is this to Jennifer and Gabe in The Maze Agency.
Sable has Captain Josh Winters in Jon Sable Freelance.

Film

Who Framed Roger Rabbit?. Private detective Eddie Valiant has Lieutenant Santino, who calls him in on the Acme murder and shows up at the end with the rest of the police. (One may come to conclusion that the Valiants and Santino graduated together from Police Academy).
Rear Window. When LB Jeffries suspects one of his neighbors has murdered his wife, he calls in his old war buddy Doyle, now a police detective, to investigate.

Literature

Harry Dresden has Lieutenant Murphy. Because of the nature of the world she rarely gets to make any arrests on cases he's involved in, but the cases still get closed... one way or another.
Ellery Queen helps his father, the chief of police. One wonders why they even have police detectives when Ellery can (and does) solve all the cases.
In addition to Lestrade, Sherlock Holmes has Inspector Gregson, with whom he is more friendly.
- Hopkins probably counts as well.
Nero Wolfe's relations with Inspector Cramer are more cordial than friendly. Sergeant Purley Stebbins, though has been known to spend the evenings playing cards with Archie Goodwin and the boys.
Hercule Poirot has Inspector Japp.
Sam Spade is friends with detective Tom Polhaus. His relationship with Lieutenant Dundy is much more antagonistic.
Lord Peter Wimsey has Inspector Charles Parker, who later becomes his brother-in-law.
In the Coruscant Nights Trilogy, the hero Jax Pavan, who "finds things" and has a rather complicated, never defined job that does involve investigation, has a slightly antagonistic but ultimately friendly relationship with Pol Haus, a police prefect on Coruscant. Of course, there's never anyone to lock up, since Pavan's enemies strongly tend to be Dark Jedi who vanish, Inquisitors working with the Empire, and Darth Vader. Mostly Haus and Pavan just meet to trade information. Haus suspects that Pavan is a Jedi, but ultimately bails him out.
In George C. Chesbro's In the House of Secret Enemies, Detective/Criminologist Dr. Robert Frederickson (Mongo to people who knew him from the circus) has a big brother, Garth, on the New York City Police Force. A much bigger brother.
John Le Carre's George Smiley has Inspector Mendel, whether nearly retired or fully retired.
McGowan from the Andrew Vachss Burke books, at least until he retires. The quote comes from a book that occurs after his retirement, so Burke cannot count on his help any more.
In the Hardy Boys books, Bayport Police chief Ezra Collig is an old friend of Fenton Hardy, though he makes few arrests himself. In a lot of books, one-off cops are typically the ones who actually arrest the criminals.
On the East Coast, Mike Hammer is friends with Captain Pat Chambers, NYPD.
On the West Coast, Shell Scott is friends with Captain Phil Samson, LAPD.
Philip Marlowe from the novels by Raymond Chandler has Bernie Ohls, chief investigator for the DA's office.
Benjamin January has Lieutenant Shaw.
Phryne Fisher has Detective Inspector John "Call me Jack" Robinson.

Live Action TV

Angel had Kate Lockley for the first two seasons.
- Lampshaded at one point in season 3. Pressed on where he got some info, Angel reveals that he hired a detective agency with some friends on the force. The rest of the team bemoans how pathetic that looks for their own agency, don't they have their own friends or something? Angel agrees it's not ideal, but reminds them that Kate got canned (mainly as a result of her association with Angel Investigations cluing her in on The Masquerade, and how bonkers she sounds to other cops as a result).
Diagnosis: Murder: Dr. Sloan's son is a homicide detective. A relative on the force.
Magnum has Lt Tanaka. He also knows an(inevitably pretty)lawyer at the District Attorney's office and several contacts at the naval base.
Marcus Didius Falco's best friend, Petronius, is a member of the Vigiles.
Monk: Lieutenant Disher and Captain Stottlemeyer.
Psych: Carlton Lassiter and Juliet O'Hara. Although Lassiter at least is less a 'friend' and more 'somewhat reluctant and slightly resentful ally'.
- Shawn's dad Henry, being a former cop until Season 5, had all kinds of friends on the force, through which he would track Shawn's case and sometimes provide useful leads/background information. Now that he's back on the force as the SBPD external liaison, these don't come into play as often.
Spenser has several police contacts.
Rick and A.J. Simon have "Downtown" Brown, a plain-clothes/undercover cop.
Jim Rockford is friends with Sergeant Dennis Becker.
Toby from The Listener has Detective Charlie Marks, though she's often inclined not to listen when he comes to her with evidence and can't explain how he got it because he found out by reading minds. Of course this changes once she finds out.
- And since she died, they found a Suspiciously Similar Substitute for her in the form of Sergeant Mc Cluskey.
Torchwood has PC Andy.
This trope + UST = Castle.^[context?]
Leverage has FBI agents Taggart and McSweeten and Massachusetts State Police Detective Captain Patrick Bonano.
Campion: Albert Campion has Detective Inspector Oates, a competent cop who often takes Albert's arrests and investigations.
Due South: Ray performs this function for Benton who has no jurisdiction in Chicago.
Tracker had Mel's on again-off again boyfriend Detective Vic Bruno. She makes use of him a few times during the series. (Trust,A Made Guy)
The BBC Sherlock version of Lestrade is much closer to this than the trope he named. He's also the *only* friend Sherlock has on the force, as every other police officer he knows would rather Lestrade not keep involving this weirdo who keeps human eyes in his microwave ( which leaves him powerless when Moriaty's manipulations convince the entire police service Sherlock's a fraud).

Video Games

Gabriel Knight isn't a detective, but he does do some sleuthing with the help of his good friend Detective Mosely. He also uses Mosely to get information for his novels.
Tex Murphy has Mac Malden.
Lewton, the Private Detective in the Discworld Noir computer game, has Nobby Nobbs as his buddy on the force. Which only really goes as far as Nobby giving him several useful pieces of gossip and convincing Vimes to arrest Carlotta at the end.
Phoenix Wright: Ace Attorney has Detective Gumshoe, who starts out as an annoyance but later becomes essentially a friend to Phoenix (while remaining an annoyance). He even helps Phoenix a bit on the sly in the second and third games, despite technically being on "the other side".
- Gumshoe serves this purpose in a more formal capacity in Ace Attorney Investigations, since he's Edgeworth's partner. Likewise, Detective Badd was the partner to Prosecutor Faraday in more ways than one. No, not that way. A different way.
Inverted Trope towards the end of L.A. Noire, where Jack Kelso plays the part of Cole Phelps' friend off the force, as an investigator for California Fire and Life.
Deus Ex Human Revolution: Adam Jensen has two friends on the force, one who you can get a favour from and accidentally get fired (although if you convinced him to do the favour you can get him a job at the company you work for...if you extorted the favour he'll just try to kill you) and the the other he does a favour for.

Web Comics

Ray Smuckles and Captain B, as seen in this Achewood strip.

Western Animation

The Swat Kats have Felina Feral, the one Enforcer who is willing to work with them, despite her uncle (who happens to be head of the Enforcers) being their sworn enemy.