If your cheap, the free demo version of SEO Powersuit's Rank Tracker seems fine, in this version you can check the top 100 results for 100 keywords at a crack and get results for the big 3, you just can't save result in this limited version.

A plus, it works on Windows, Mac and Lenux

yea rank tracker is great but for a fast and simple program free google moniter is a good one too

I use SESCOUT.com really good tracker, sometimes a few problems but is usually is sorted within hours.

They offer daily updates of your rankings and you actually see your keywords going up and down in graphs etc...