How come Dropbox gets an icon in Finder sidebar?
As you've noticed, recent versions of OS X only display generic folder icons in the sidebar. So why does Dropbox get special treatment?

The short answer is that Dropbox uses undocumented API to accomplish this. In non-technical speak, it's a special hack that's installed by the Dropbox application.



Some curious folks on StackOverflow found the specific mechanism used by Dropbox: good ol' mach_inject. The same bundle provides both the toolbar item and the sidebar icon.

If you're curious, the resource files live here (at least, on my system):

/Library/DropboxHelperTools/Dropbox_u502/DropboxBundle.bundle/Contents/Resources


Clearly, Dropbox goes to great lengths in order to integrate seamlessly into the Finder. Lacking similar treatment, other ordinary folders have only generic icons in the sidebar.



Edit 10/1/2015: As of Yosemite (10.10), there is a new, sanctioned Finder Sync API for integration, as doovers points out in another answer. In El Capitan (10.11), System Integrity Protection will not permit the old mach_inject-style approach.