For UKGuest.com I tested in IE6, Firefox, NS7 and Opera on the PC and other people were kind enough to test for me on IE5.5, NS7, Safari and Firefox on the Mac.

Not worth checking in IE6, Firefox AND NS8 since NS8 is just Firefox and IE in one - which makes it great for testing in both of those.

I started testing in IE5 and IE5.5 on PC but they don't handle CSS at all well so I had to bin them off and be happy with compatibility across at least 8 browsers on 2 operating systems