As stated above, both unrar and unzip are found in extras-devel (even though I haven't had any problems installing these rather small command line utilities, there is always a risk involved in installing from this repository, so do it at your own risk).

I also found out that the xarchiver GUI frontend in the diablo extras devel seems to work out of the box on the n900. You can find it here:
http://repository.maemo.org/extras-d...emo2_armel.deb

But again, even though the installation of this hasn't caused any problems for me, it might not be the case for you, so do it at your own risk.