Scanlation
Fan-produced translation of a foreign-language comic, produced by scanning individual pages into graphics files and then replacing the original text with vernacular translations. The paper/jpeg equivalent of Fan Subs.
A good Scanlation requires a fair-quality scanner, and a good eye for which fonts are obtrusive and which are not. It's also important to carefully match the background shade when covering the untranslated text - a bright white box on a medium-gray word bubble can be painful and distracting. The best scanlations can come close to being indistinguishable from an unaltered manga page (or, in other words, close to a professionally-licensed manga page).
Some scanners of Hentai Manga and Dojinshi dispense with the "heavy lifting" job of actually translating foreign erotica and simply write their own scripts to match the action - dubbed a "Rewrite". This can occasionally produce something more amusing than the original, especially if it's a Gag Dub-style "translation", or an explicit parody of bad rewrites. This does not work well with Porn with Plot manga, obviously.
A major clampdown by publishers in the Summer of 2010 resulted in many Scanlation aggregator websites such as MangaHelpers and OneManga closing up shop... bad news if the manga you like are No Export for You, licensed but out of print, or excessively delayed!
See also Fan Subs for the Anime version and Fan Translation for the video game version.