Remote Desktop Connection for Mac
Microsoft
RDC
Never thought I'd be saying I couldn't do without a MS product, but short of opening up a browser window on another monitor connected to an XP box (or flipping an A/B box), I just log into my XP box from my Powerbook to view web sites side by side to see site development in a real MSIE. (Safari debug doesn't come close enough to be real)
Keeps me on my Mac, but lets me develop for MSIE... add Exposé and you can really speed through comparing subtle, but critical difference in CSS.
G.