YubNub is an online command-line community extensible search engine, or as they say "a (social) command line" for the web.

Some commands which may be of interest to the community are

ltu - Lambda-the-Ultimate.org

scholar - Google scholar

cs - Citeseer

oeis - Online Encyclopedia of Integer Sequences

You can also easily create your own commands. Please share any new commands you create or find which may be relevant for the community.

[Edit: Removed "plre - Programming Language Research Engine" from the main list, since it was criticized as being self-promotional.]