If any wine developers read this, great job. You guys have done an excellent job.
The program that could completely destroy one of the major barriers to Linux's marketability is wine. Being able to run Windows programs no problem under Linux is amazing. It's not perfect yet, but many programs already do work. Personally, I've only really tried games and there are several ones that work. Starcraft (mostly), Warcraft III and Diablo II are the ones that I can remember at the moment (funny how they're all Blizzard games). Diablo II even runs faster than on Windows. Many of the newer ones have problems, especially ones that depend on the .NET framework. I've also never managed to get a program published by Microsoft to work (odd).
Another great thing is their updates are very frequent (and they have an Ubuntu repository), usually being around two weeks. So sometimes you'll have a bug with a game and be used to it, but then all of a sudden it's gone. Awesome!
A note to those who have problems with multi-disc installations: if you're using command line, do not browse to the CD's folder. You won't be able to eject the disc. Instead type 'wine /media/cdrom/setup.exe' from outside the folder. Then to eject the disc, you can eject it normally or type 'wine eject'.
EDIT: Some more games that work: Battlefield 1942, Oblivion (except for some graphics glitches here and there).