Have been using this one for a while and I think it is good. You can actually set up an API key for each client and give a link for them to use and check where their site is for various terms.
I've found keyword tracker at http://www.digitalpo...tools/keywords/ useful for tracking multiple keywords and pages. You need to get the Google API licence key to use the service.
They will eventually get around to charging for this but it could be worth a few bucks, maybe worth more if you could get an agency deal and then have it private labelled as part of your members area.