I found a really useful extension for Firefox: Window Resizer. If you do any kind of web development you should already be checking to make sure your content looks good in different resolutions. Nothing screams 1990's like a fixed width, "Best when viewed in 1280x1024 resolution" website. I'm rather anal when it comes to making sure my websites are as good looking on one computer as they are on another; why should I put all the time and effort into making a website look good if it only looks good on my computer?
But of course I need to set some kind of standard, so I used my StatHound visitor resolution statistics to determine what the lowest common resolution is for visitors to this blog -- it happens to be 1024x768. The Window Resizer extension is useful because it allows me to quickly resize my Firefox window to see what my website would look like in that resolution. It's a lot easier than changing the resolution on my monitor, maximizing Firefox, and then changing the resolution back to the original.
I also believe content should be readable from any web browser -- again, it's so 1990's to say "Best when viewed with Internet Explorer 6+". I actually test my websites using Firefox, Internet Explorer, Safari, and even the text-only web browser Lynx. I consider a text-only web browser to be the ultimate test for readability and that's one of the reasons why I frown upon flash-only websites, or websites that don't provide a "text version" option.