I use a Sports Instruments ECG-3. I've had this one about 3 years now, which is longer than the Polar M21 that it replaced. The Polar was flaky the whole time I owned it and completely stopped working after about 18 months. That one cost me about $120 and so when I replaced I didn't want to spend much (I tend to break and lose watches a lot). I spent about $50 on the ECG-3 and it works great. The band is not the best design, but the watch itself has been fine and has taken a good beating. I wear the HRM year 'round on all most trips (hiking, skiing, climbing, riding, etc,) and and many of my races in the spring and summer, so it gets it's share of abuse. My main complaint with it is the button on the side is too easy to hit. I wear my watch on my left arm and with my hands bent back in a normal riding position the top of my hand will press the button, which cycles through the modes. It's mainly an annoyance and wearing it on my right wrist solves the problem.
Edit to add: I got my Sports Instruments at Bike Nashbar on clearance, I think it's normally more like a $75-$100 unit.