git bisect, ccache, cowbuilder
Git bisect, ccache and cowbuilder: a combination made in heaven! Tracking down a commit which introduced an ugly bug with those tools was a breeze.
Git bisect is very useful finding a commit which introduced a bug very quickly, ccache massively reduces compiling time. Compiling icedove (thunderbird) on my laptop using cowbuilder takes roughly 30 minutes. Using cowbuilder with ccache, it only takes 10 minutes, where most of the time is spent setting up the build environment.