I messed up my Gentoo installation. Not wanting to compile everything again, I searched for options on how to repair it and got some hints on the Gentoo forums.

First, there is:
emerge -e world

If i understood it correctly, option -e hides all instaleld packages from emerge, thus emerge installs everything again. If you do this, be sure to have a two day vacation or a life beside computer hacking.

And then there's a very useful package, gentoolkit. emerge gentoolkit adds the following tool:
revdep-rebuild -p

This tool checks all packages for broken dependencies and passes them to emerge. All added options are passed to emerge also. Using the -p (--pretend) option, you can find out beforehand, which packages are corrupted.
In my case several distfiles (the downloaded source archives) were corrupted too. To download them again, they simply have to be deleted.

It seems revdep-rebuild doesn't check ALL dependencies though, at least that was my impression. I had to emerge several libraries manually to make things work. I'm still emerging, but hopefully my Gentoo will be back soon. In the meantime I'm using Ubuntu, which arguably is one of the best distributions for Linux newbies but doesn't come close to the freedom of Gentoo.

If I can't make Gentoo run again, I'll probably check out Linux from Scratch instead of installing it again though. emerge would be missed of course...