I noticed this
SERP Tracker not so long ago and it looks interesting.
I haven't used it to test the waters yet and how well it performs but it looks promising.
The thing what puts me of using these type of tools is the discretion of these solutions being offered.
Let's say you have found a very nice keyword to target related to a high value niche and you can make some good money of it and exposing my profitable keywords to SEO savvy Marketeers is not something i would like to do of course.
This is a key issue in offering a good service, some sort of set up that guarantees me that my keywords are not being monitored by anyone else then myself.