WEBoggle is a nicely-done online Boggle. Your score for each timed round is ranked against other players, and the words you missed are listed, along with the words everyone missed. Clicking the board rotates it.
Aside from missing the less-obvious, rare words, I have some difficulty finding pesky little common words. Humbling, but fun.
You can choose between 4×4 or 5×5 boards and the game is alwyas on and available.