Pick of the day, ccache
I recently tried and adopted ccache, which (quoting its webpage) acts as a caching pre-processor to C/C++ compilers, using the -E compiler switch and a hash to detect when a compilation can be satisfied from cache. This often results in a 5 to 10 times speedup in common compilations.
I must say I was sceptical, but ccache really works great, specially when you have to compile again and again the same code, like a Debian package which you have to clean but which source code remained the same.
The only glitch is installation. If you wish to use it by default for all your compilers, there is still some manual configuration. I'd really like it to provide an alternative to, say, cc.