Being of a more biological bent, I almost invariably choose my nicknames from scientific names. I find that this gives you an interesting and often unique presence on the 'net. It also has the added benefit of allowing you to easily choose a fictional first and last name for those noxious web sites which insist you provide them with personal information for their own profit, rather than your benefit.

Here are some examples of names I have used in the past:

And here are some that I have never used, but might in the future 'cause I think they're cool: