Undoubtedly, the best online tool for tracking keyword ranking in the Google SERPS is DigitalPoint's Keyword Tracker tool. You can find it at:
www.digitalpoint.com/tools/keywords . Note that you must have a Google API key to use this tool. If you don't have one yet, you can get one for free at Google API Key.
DigitalPoint's keyword tracker tool tracks changes over time not only to your keyword rankings, but also to the number of backlinks to your pages as well as the PR (PageRank) of the pages.