Owl City/Heartwarming

Many of his songs but mainly, Vanilla Twilight,

"Oh, if my voice could reach through the past, I'd whisper in you ear: 'Oh darling, I wish you were here.'"

- If My Heart Was A House

If my heart was a compass you'd be North
Risk it all cause I'll catch you if you fall
Wherever you go
If my heart was a house you'd be home

Air Traffic.
Meteor Shower also counts.
Strawberry Avalanche always makes this troper so incredibly warm and happy~.
Hot Air Balloon, I know I can't be the only one who thinks its heartwarming...
The Bird and the Worm. This song always puts a smile on This Troper's face.
"To The Sky" is an uplifting heartwarmer, especially if you've read Guardians of Ga'Hoole (but even if you haven't.)
Sure, they can get a bit too sweet, but This Troper can't be the only one who does get some serious warm fuzzies from some of his music. Special mention to "If My Heart Was a House."
Speaking of heartwarming, Taylor Swift's song Enchanted is a sweet song about meeting Adam and being, well, enchanted. The Valentine's Day after it was released, Adam posted his own version of the song on his blog, directed to her. The heartwarming is pretty well summed up by his tweet about it: "Dear Taylor, everything about you is beautiful. I want you to know I was enchanted to meet you too."
An example from All Things Bright and Beautiful, Honey and the Bee is a pretty sweet song. It makes this troper just look out the window listening and smiling. Kudos to Adam for setting up these two lonely individuals and then bursting into possibly one of his sweetest, most beautiful songs yet.
And these lyrics from Plant Life:

If I were to pluck on your heartstrings, would you strum on mine?

The whole of Vanilla Twilight is naturally one, but the last verse in particular always gets this troper when I listen to it:

And I'll forget the world that I knew, but I swear I won't forget you/Oh, if my voice could reach back through the past, I'd whisper in your ear/Oh, darling I wish you were here.

Tip of the Iceberg may have one set of creepy lyrics, but it is honestly sweet and heartwarming. From the Determinator narrator braving the ice and snow, to the final set of lyrics, which are honestly just beautiful:

snow drifts build up and enfold us/as we wait out this winter storm/so we snuggle close in the darkness/and keep each other so warm...

"Peppermint Winter." He manages to capture everything we love about Christmas, and set it to a beautiful melody.

This peppermint winter is so sugar-sweet, I don't need to taste to believe, what's December without Christmas Eve?