In spite of fxtr's rather odd post suggesting the same firm you're looking for alternatives to, this is a good question. If you're trading futures, there are quite a few alternatives to IB, Infinity Futures knock spots off all other brokers I've ever used (which includes IB). I can't comment about Forex as I don't trade it. For stocks, TradeStation might be worth looking at (although their customer service isn't that great either in my experience), and thinkorswim appear to be popular with options traders.
I may be wrong about this, but I believe IB are one of the few brokers who offer a universal account from which you can trade just about anything. If you're looking for that level of flexibility, I suspect they'll be tough to beat.