If you're looking for an easy way to monitor and track your ranking for specific keywords in Google then you should consider using GoogleRankings.
Their most basic function allows you to see where a single site ranks for a single keyword. My guess is that you already know where you rank for the terms you optimize for though. That's why their advanced rank checking functions are more effective for gathering actionable data.
With the "Mass Keyword Search" you can search for up to ten keywords for one site. The "Mass Domain Search" allows you to search up to ten domains, though only for one keyword. The only thing they're missing is an autosearch that emails you your results every morning so you can see just where you stand.
Their Advanced Search function allows you to really target what kinds of pages you want to appear. You can limit for file formats, languages, place search term appears, by domain (good for massive sites), and by last update (cool!).
The GoogleRankings site also offers a keyword density analyzer and a keyword selector. The selector simply shows you which words appear the most frequently on your page though, which is hardly a way to determine the best keywords for your site. Still, it's a good place to start, and might give you an idea you hadn't thought of yet for keywords.
Their help file is extensive and despite a spelling error or two (Hungarian web developers built this site) is understandable, professional and written with the SEO in mind.