Last night, I was reading Caches and Maps in Terracotta by Alex Miller when I came upon his link to the JPC (a.k.a. JavaPC) Project. It sounded familiar and I decided to follow the link. It turns out to be an open source project implementing an impressively functional and performant x86 emulator in Java. I had fun last night launching an MS-DOS environment within an applet (what a concept!) and playing Donkey Kong for the first time in ages. According to the site, the emulated layer runs at about 20% of your processor's speed - not bad for a emulator written in pure Java.
Note: After my first visit to the site, it seemed to be having problems. Just in case it had to do with visitor load, I changed the link above to point to the copy in Coral Cache. The site is now totally back up and behaving normally. If Coral has any problems or is slow, you can safely go to the site directly.
Update: I got an email from one of the JPC team members saying that they'll be at JavaOne this year with some new stuff. I wish I could be there to see it. I guess I'll have to wait for it to show up on their site.