A dictionary developed at the Cognitive Science Laboratory at Princeton University.

It is freely available for download and use from the Internet.

I find it to be more up to date than Webster 1913 (well, duh). But the archaic words are missing, and the descriptions are less ... colourful. It does, however, have various descriptions of computer related topics, which makes it an excellent supplemental to Webster 1913 (or any other infrequently updated database).