Google Analytics:

Google also created a free cross platform conversion tracking tool which comes
with their AdWords product. It works for tracking email ads, banner ads, and
various PPC search engine ads.

On top of the free built in tracking feature Google also bought out Urchin and
rebranded the tool as Google Analytics. Google Analytics is free, and perhaps as
full feature as any other conversion tracking tool, but if you use it you are sending
your valuable market data back to your source of traffic. That may work against you
if they use your best converting terms and most frequent referrals to recommend
keywords to your competitors.