If you haven't heard about it yet, there's a new Quake on the block:
www.quakelive.com
It uses the old Quake 3 engine, and runs from the website. Takes minutes to install and is very fun. It's not a high-end graphic game, but runs very smooth. Still in beta, so the site may be down from time to time. They do keep updates on Twitter.
My username is doubleminor if you want to add.