another vote for the MI Audio Crunchbox for higher gain 'Marshally' tones.
for lower gain stuff, the Blues Deluxe (or whatever it's called) by MI Audio is really nice.
if you can find an old Matchless valve stompbox, those were really tasty and well worth the cash - I should really get off my arse and make a valve overdrive, seems to be a market waiting for those things if the price is right.