Problem: My wife and I both use different iTunes accounts. She’s bought things and I’ve bought things; now how do I play something she’s bought on my computer or my iPod?
iTunes purchases are keyed to a specific iTunes Music Store account. iTunes lets you authorize up to 5 computers per account by going to Store->Authorize Computer and entering the Apple ID and password of the person who made the original purchase. Once you’ve done this, any purchase made with that Apple ID will be playable on that computer and any iPod that syncs with that computer. So I authorized my computer with her account and hers with mine, and we both can play tracks that the other has purchased!