Coyote's Guide - Coyote's Guide to IDL Programming is a resource for people who want to learn more about IDL. This site is very useful and the first place I go when I need to find the answer to an IDL-related problem.
IDL Astronomy - This site hosts many small but useful procedures written in IDL. Their intended audience is programmers working in the astronomy field, but I often find procedures that are useful for medical image processing.
IDL Documentation - IDL Documentation in HTML Format from the Department of Astronomy at the University of Virginia.
Markwardt IDL Library - Another collection of procedurs written in IDL. I haven't used this site as often as the Coyote Guide, but I like to keep it on my list just in case they have something I need.
comp.lang.c FAQ - Frequently Asked Questions from the comp.lang.c newsgroup. A useful site with lots of commonly-encountered problems and/or questions.
Programming in C - A collection of lots of links that are useful to the C programmer. The site is very old, and hasn't been updated for many years, but many of the links are still good.
IOCCC - The International Obfuscated C Code Contest. This site could only be appreciated by a C programmer. Each year a contest is held to determine who can write a functioning C program using the most obfuscated code possible. Entertainment value aside, this site is mostly an example of what not to do.
C++ FAQ - A 14-part collection of frequently asked questions about C++ programming.
Learning with C++ - A full textbook about programming with C++. It is part of the "How To Think Like A Computer Scientist" section of the Open Book Project.
Winsock Samples - Sample projects for programming Windows using Winsock. Somewhat old now, but it covers the basic protocols and happens to use one of the Microsoft Visual C++ versions that I own.
inpout32.dll - inpout32.dll is a DLL for Windows that makes interfacing with the parallel port easy. In Windows 98 it was easy to access the parallel port, even in BASIC (or, more importantly, VBA). This all went to pot with WinNT, but this nice little DLL solves the problem completely.