PowerTap is a good option.

If you look hard, you can find good deals on used SRMs.

Power2Max has lowered its prices a ton, putting them in the range of Stages, and they get really solid reviews.

Some people like Stages, and they can be had cheap. I've heard less-than-stellar comments about them from people who really care about the reliability and accuracy of their power data. But if you're not looking for a pro career, it should be sufficient.

And then there is still Pioneer - I've heard mixed things from people about the reliability of them, Garmin Vector (the newest version supposedly fixes the installation issues), BePro, Brimm Brothers, and Verve Infocrank - of which I've only heard good things..