James
Bond Meets The 7 Layer OSI Model
The best explanation of the OSI/RM I've seen yet.
Howstuffworks Computer Channel
If you need help visualizing how things like hardware, software, routing, or firewalls work, try this site.
Subnet Mask Cheat Sheet
Once you understand basic subnetting concepts, this cheat sheet can help you work through the details.
IP Calculator
Input your network number and subnet mask, and it will do the rest. But you'll need to already know a bit about subnetting in order to interpret the results.
ProProfs
An online academy where users upload study guides, quizzes, and other resources they've created to help themselves and others get certified.
Internet Standards Organizations
ISOC's guide to the relationships between the different standards organizations (IETF, IANA, and so on).
Power Supply Calculator
You input your computers components and it tells you what wattage you'll need from your power supply.
S.M.A.R.T. is Dumb
Some time ago Google cooperated with Stanford on a study showing that the Self-Monitoring, Analysis and Reporting Technology built into modern hard drives is not usefully predictive of failure. Here's a transcript of the Security Now! podcast where Steve Gibson discusses the study.