As far as more mainstream Linux's go I've been running Red Hat 9 for about six months now. Unfortunately Red Hat is ending support for it, boosting their prices to the sky, and generally showing everyone that they only want to support large enterprises not individuals. Sooo...
I'm thinking maybe Suse Linux, which seems to be going after the $69 market that Red Hat is abandoning. I'm also considering going to FreeBSD.
Opinions?
If you like RH 9, try out their new freebie,
Fedora. The stable version is Core I, but Core II is coming out (hopefully) this April.
It's built on RH9, and everything will be familiar to you. In fact, RH uses Fedora to beta test it's Enterprise software, I believe.
I've been running FC1 on my Sager 8890 since September, and have had no regrets. It interacts beautifully with my work's Windows network, and I can interchange documents with Windows users, using OpenOffice 1.1.