I personally have used Courier IMAP for a while, first on my SuSE server and now on my Gentoo server. Some nice multiple domain Howtos exist, as well as Gentoo specific ones.

My main reason to go with Courier was the support for Maildir. Others have support for it as well now, but back when I initially looked, it was the suggestion for a server with support for it. Just make sure to pair it with a filesystem that likes lots of tiny files, like reiserfs. Oh, and if you go with Courier, get used to the political fight between them and I think the UW folks. It's about as bad as the fights between Debian users and RedHat users.