The web now hosts a huge number of reference works on just about any topic, so The Dictionary of the History of Ideas might not be the most amazing thing one can find online. But it is useful, so you might want to bookmark it for future use, even if your use of it is sporadic.
Take, for example, "The Faust Theme". It isn't as much fun as thumbing through a big, heavy book, but it can be useful for filling in gaps in one's learning.