There are two comparisons that I find most interesting. First of all, when we look at processor architectures, it's pretty clear that Intel has more or less taken over the entire market, with a massive x86 dominance. I can assure you that predicting today's x86 dominance in the Top 500 list in 1997 would've won you a talk with the men in white coats.
When we look at operating systems, it becomes quite obvious that Linux has taken over the Top 500 list. AIX powers 21 machines, and Windows HPC 2008 does 5 of them. There's an OpenSolaris machine, and a Super-UX one, and the rest is all Linux (or at least partly Linux, such as Cray's UNICOS/lc).
So, the supercomputing market is more or less standardising on x86 and Linux - exactly, commodity hardware.