I use peat for seed starting and sometimes in pots. It's cheap, and it fills a pot, so I use it. I always mix it with plenty of vermiculite or worm castings or something to hold the moisture. There are serious environmental concerns though, like Cynthia said. But I do weekly penance for that
Another fertilizer I like for container growing is steamed chicken manure pellets. It's usually more balanced than most fish emulsion (the type I get is 3-3-3) and wont burn. I've had great success using it in my transplants. One small handful per gallon pot goes a long way, it seems. Unfortunately it can be hard to get a hold of..I got lucky. I live close to the manufacturer and pick it up for 6 bucks a 50lb bag. Bwahaha!