The OS X port of XScreensaver is simple, stable, and unobtrusive as far as I can tell. Unfortunately you can’t (easily) pipe program output into Phosphor and other textual screen savers, but other than that they’re spectacular.

I did toss out some of the less impressive ones, though. Here’s what I kept installed:

Apple2.saver
Atunnel.saver
BSOD.saver
Barcode.saver
BlockTube.saver
Braid.saver
Celtic.saver
CubeStorm.saver
CubicGrid.saver
Epicycle.saver
Eruption.saver
Extrusion.saver
Fireworkx.saver
FuzzyFlakes.saver
GLMatrix.saver
Halftone.saver
HyperBall.saver
HyperTorus.saver
Interaggregate.saver
Interference.saver
Kumppa.saver
Noof.saver
Phosphor.saver
Piecewise.saver
Pipes.saver
Polytopes.saver
Pong.saver
Pyro.saver
SpeedMine.saver
StarWars.saver
Starfish.saver
StonerView.saver
Substrate.saver
Swirl.saver
Wormhole.saver
XJack.saver
XRaySwarm.saver
m6502.saver