Know of any good portable ones? Good can mean useful or fun, I particularly like 3D ones like Flurry (as seen on Macs), Fireflies and, probably my most fav of all, SkyRocket from ReallySlick.com (Which has lots of cool screensavers). Sadly, I don't think any of these are portable (to be fair, I've never really tested them).
Of course fun can also mean prank... as in BlueScreen from SysInternals.
Portable Screensavers...
If you like Direct3D-driven fishies and stuff, give this one a try. With my little redirector dll you can create a standalone portable version. And the only file it uses is the scr so it's completely portable. You can run the demo for free for as long as you like but the real thing costs some.