Restricting plugins to a single site

Sort of a basic question, but what's the best way to make plugins available exclusively to one or one group of blogs? Right now I have one blog that will function as a database, another that will be used for products, so there are certain plugins I wouldn't need on both. On the product blog, MarketPress would be awesome, but it isn't needed on the database blog.

On a similar note, if I wanted the membership plugin to cover content access on both blogs, what's the best way to go about that?