circuitben

Reference

Electronics

alldatasheet.com
A huge semiconductor datasheet archive. Has almost everything. If you want to know about a chip or transistor, go here.
On-line Verilog HDL Quick Reference Guide
A good summary of Verilog syntax
FCC ID Search
Look up information on most electronic devices. Sometimes you get schematics, but these are ususally not published anymore.
Using SPICE with gschem

SMD topcode/topmark (package marking) databases

Use these to identify those SOT-23 parts with only two letters on top.
Lists at TKB-4u.com
Franz Hamberger's list
Fairchild
Maxim

Software

STL Reference
SGI's documentation on the Standard Template Library. Don't use C++ without it.
Boost libraries
Lots of good libraries. Some of these are as important as STL.
Schneier on Security
Bruce Schneier's blog on security issues.
comp.risks
Forum On Risks To The Public In Computers And Related Systems.
Ada 95 Language Reference Manual
The specification for the Ada language.
SDL Documentation
SDL is a simple way to do cross-platform graphics, sound, and input. Doesn't have any drawing primitives, though.
Ada 95 Tutorial Python string methods

Other

ASSIST Document Search
U.S. military standards and similar things. This is where to get MIL-STD-xxx documents in PDF form.
NPA NXX lookup
Find out where a US/Canadian phone number goes