We always tell these people to get metronomes, but few people actually know where to find one outside a crappy First Act one at Wal-Mart. I own a Boss DB-30, but it was $40 which most people don't want to pay.
Here's a few free online resources:
www.metronomeonline.com/https://webmetronome.com/ (I find this the second link to be the best, it let's you accentuate beats which is good for complicated time signatures and it has a link to a tuning keyboard.)
There's also an iPod/iPhone app called iTick that works perfectly (also free).
Anyone want to sticky this topic? Would definitely be helpful for those who might not be sure where to get one.