Yea. Each time I build a computer or make a major change (RAM, CPU, motherboard, overclocks), I run my own set of stability tests.
I use the following simultaneously:
-Prime95
-Winamp 5 set to play random songs in a 1000 song playlist
-3DMark 2001 SE set to loop the benchmark infinitely
I do that for about 12 hours. Then I close 3DMark and use the computer for a bit, which involves surfing the net, e-mail, Paint Shop Pro 8, and games (GTA3, NFSU, Tribes 2, FarCry). Next, I launch Tribes 2 and host a server with 6+ bots and observe the game for several hours (minimum of 4 hours)...T2 is coded pretty bad and tends to crash on seemingly stable machines, so it's a fairly good indicator of stabiltiy. By this time, it's usually been at least 24 hours of straight Prime95 testing with Winamp and many other programs, along with 16+ hours of 3D testing. If it passes all of that, I declare it stable.
I think that works pretty good.