Just about every foundry's website has a different spin on type testing. One that I found interesting (although I ultimately dislike it) is Wilton Foundry's FontGenerator (click on "View this font in FontGenerator), which displays the sample out of moving particles.

Fonts.com's "Free Text," while clunkily implemented, gives possibly the most flexibility I've seen, allowing you to specify any point size you want as well as the text and background color. It doesn't seem to kern, though.

EDIT: Reminding myself of your original question, for me, some way to see the letters up close and personal is extremely important, as I by typefaces for both their readability at normal sizes and their quirks at larger sizes. If a site doesn't have a good type tester, I look for a PDF that I can zoom in on. Lacking both of those would make my decisions more protracted.

There is a website called moretype.co.uk which has an excellent type tester.

http://ourtype.be has a very good type tester.

Thank you!