Okay, we have a bunch of online museums for you today.
The American Sign Museum has a collection of really cool, well, signs. Layout and navigaiton is weak though.
The American Museum of Beat Art could be thicker on the content, and could be laid out somewhat better, but I still like it.
The American Museum of the Moving Image is a well laid out site, pleasent to browse through. The sizing is a bit small (it's in desperate need of a fluid layout). But some good stuff.
The American Museum of Natual History: Congo Expedition is the best of the lot in terms of layout and browsing experience. The Gallery holds a lot of fun.
Speaking of fun, the World Museum of Erotic Art is defintely different. (Probably NSFW)