You could use ~tires~ :~D
Oh dear...kinda true, though. Except that they aren't really quite the right shape...
not a bad idea... mrsanad is plantin' taters in muh old tractor tires... they work well for stuff you plant in mounds... squash, zucs, cukes... you could use 'em for other things too... just gotta break outta the "row" mentality... pole beans steaduh bush beans... you get the picture...
check the yellow pages fer a place that sells concrete blocks... you can usually get seconds (off color) for less than a buck... a 4'x16' bed stacked 2 high takes about 72 blocks... so yuh might wanna go one high tuh start...
FWIW, I like hair's idea of a "summer hedge" of corn... plant at least three to four rows deep for pollination purposes... I wouldn't waste the precious space in a raised bed on corn... but thats just me...