JossMouette: Another example that comes to mind is the BTS usertags. Although there is no convenient interface to browse them, they are already used for many applications, my favorite being bts-link-upstream - another tool we've started using the day it was implemented.
JossMouette: I should also mention CIA, which dumps all SVN commits on #debian-boot and #gnome-debian. This is extremely useful to follow what's going on without flooding a mailing list that's hard to read.
Steinar: "Changes in reportbug quickly reflect in bug reports" NathanaelNerode: Reportbug is simply quicker for me to use when filing bugs than other tools, and its template is helpful. I report a lot of bugs, often for QA reasons, and I've taken to using reportbug every time.