I was using training peaks, but checked out beginnertriathlete.com and it's WAY cool. You have your own little blog and you can log everything you want, from heart rate to hours slept, weight, resting HR, etc. etc.

Then the really cool part is that other people leave you little notes to inspire you. (Although you can make your log private if you choose.) Practically every day I have a note that says "good workout" or "nice bike." All of this stuff is free.

If you pay for membership, you can get more advanced training plans geared to triathlon, but you certainly don't need to pay (I don't).