Remember WEBoggle? The guy who made a mirror has ended up making tons of functionality improvements. It’s apparently a lot faster on the backend, you can submit words that you think are missing from the dictionary, and it now supports two simultaneous games (one of which is 5x5 Big Boggle)! Check it out.
Update, 08-13: Holy hell. WEBoggle was posted to Metafilter today (for the second time, to be honest). It was flooded with about six thousand players. I wish it were still on my site, but it’s nice to see comments like “This kicks ass. Best use of simple DHTML game I’ve seen."