Stages is a good option and won't require new wheels or a whole new crank.

Power2Max looks interesting if your crank is already compatible.

iNewton is also intriguing. Lots of doubters out there but the results seem perfectly accurate. Of course, at that price you may as well get a strain gauge based meter.

PowerTap is a nice option too if you feel like either building up a new or buying a new rear wheel. Prices have plummeted in the last two years.