Tool to install fonts on demand
Theres a few really good font managers out there that offer you this capability.

Suitcase Fusion's been around for a long time and is really popular, personally I find it a bit bloated though.

Fontcase is probably the most similar interface to fontbook and is also one of the most popular. It used to be my favourite, but i've found it buggy in ML.

Font Explorer X is again popular and has been around a while.

Type DNA is an interesting new contender, it was one of the first apps to get it's plugins working with CS6. They're also offering 6 month free trials to students.