Most people probably don't have three or four computers with multiple OSs loaded for webdesign purposes. Thus, it's hard to get an idea of what your page looks like in different browsers. Even if you designed it with strict w3c standards, it might look weird due to some odd browser glitch. I was browsing around today and found
www.browsercam.com
They give you a free 8 hour trial (or 100 screenshots). Their service basically lets you take any webpage, select as many (popular) browsers as you want, and have screenshots of them created in seconds. I was using it today to test out IE5 for OS 9/X and some Linux browsers.
[edit] I attached a screenshot of the thumbnails that browsercam generates [/edit]