I use reiserfs for maildirs on my office network. It's very quick and is probably the most efficient fs for small files (except for reiser4, too new for my tastes).
Only thing I don't like about it is when setting up a new filesystem (mkreiserfs) it doesn't check for bad blocks. You have to use the "badblocks" command to make a file containing the bad block mapping, then feed the file to the reiserfs tools. I've no idea how bad blocks relate to a Linode.
Update 4/29/08: John Dong (jdong) has enlightened me regarding the waning support of ReiserFS. The website of the developing company, Namesys, has now gone offline. OpenSUSE did have good reasons to stop using it as the default filesystem, and I wished I'd run across their reasoning earlier (link below). I'm reevaluating, and will probably start using Ext3 instead.
http://blog.linuxoss.com/2006/09/27/sus ... efault-fs/