Here are some websites that cover the gamut of pre-algebra and high school math. Of course, this list could go on and on... but there's only so much room here.
Jefferson Lab - Great practice site that gives students feedback about their answers.
Purplemath - What makes this site really great is that not only are there descriptions and examples, but you can search for topics, you can look at topics in more detail, and there are images as well. This site has middle school and advanced topics (high school and college-level). As an instructional designer and teacher, I have to say this site is very well laid out.
Algebra Online - You can find calculators here, lessons, tutorials, and some examples. Not a bad site, but don't rely on it either - you still have to know what you are doing and you still have to practice.
MathForm at Drexel - Asking "Dr. Math" a question can take awhile to get answered, but you can peruse previously asked questions and get some pretty good explanations. Again, this site has advanced topics that are described, as well as more basic questions. The answers can get detailed and long, but there is plenty of information and topics to keep you interested.
Texas TAKS Tests - Texas has been using an end-of-course assessment longer than VA has, and this site lists some of the released test items. Grade 8 math is Pre-Algebre and Grade 9 math is Algebra I. Even though states may differ in how they teach content, math is still math and this can't hurt to use as a practice site. The practice test items are available in a PDF format or in an online version.
8th Grade Pre-Algebra Formula Sheet - The state-approved formula sheet used by 8th graders (this is in PDF format).
Algebra I Formula Sheet - This is the state-approved formula sheet for Algebra I students
Algebra I End-of-Course Practice - This is from Prentice-Hall, the textbook publisher, and is an online end-of-course assessement.
8th Grade Textbook - This is the publisher for the 8th grade textbook - great practice for pre-algebra and even Algebra I.
Math.com - Like Purplemath.com, this site lists topics that can be examined with examples and descriptions of the process.
Here are some websites that I think are particularly interesting - whether for learning, having fun, listening to music, keeping up with the news, etc. As much as I value education and love math, I really do have other interests as well - and these are some of the sites that I feel could be of interest to others.
Pandora - You can create your own music station here - this is excellent! You can create up to 100 stations, so you can't get bored. Music is great for the soul.
WAMU - DC's Local NPR Station - I'm curious at heart and I like to hear different views (even when I vehemently disagree), and WAMU has a great lineup of talk shows. This is DC's local NPR station so some of the shows are national, some are local. I go to this site just about every day.
VA Tech - After changing majors a few times, I finally ended up with a B.S in Environmental Science, stayed in town after graduation and then went back for a MAED in Instructional Design. I still do some instructional design on the side in addition to teaching, and next year I'm going to get back into some work on environmental issues as well. I also use this site to keep up with the sports teams here.
Monk, Psych, Burn Notice Online - I am not a huge TV watcher - get bored and I don't have the time, and I don't always get to watch my favorite TV shows, but when I can, I love watching Monk, Psych, and Burn Notice on the USA Network - and you can view some episodes online. NBC.com shows classics like the A-Team and shows like Scrubs and The Office, and ABC.com has Ugly Betty. The internet is great for catching up on some favorite shows.
Get Outside - It's Good for You! - Even though I'd rather be living near the beach (I'm from the Tidewater area, VA Beach to be exact), I do like the parks in NOVA. Leesylvania is near us, as well as Mason Neck - both great for picnics, walking the dog, kayaking, hiking, or just "getting out". Prince William Forest Park is great, and the Northern VA Park Authority has some great parks that aren't far away. Being outside, even just reading a book, makes you appreciate life - even if you're feeling down. Also, it makes people appreciate how fragile our Earth really is - so get outside! We only have one life to live, so why live it inside?
Popcap Games - Play for Free - Everyone needs a break, even if just for a few minutes (or hours!), and here's a site where you can play addictive games online for free. I got addicted to Zuma and had to quit cold turkey:)
Artpad.com - Creating Artwork Online - Creating artwork online and you can save this, put on frames, and just explore your creativity. Everyone's creative!
Podcasts - You can create your own podcast very easily, or you can listen to some already created.
Math and Math-Skills with Java - This is the National Library of Virtual Manipulatives site where you can practice math skills with games and interactive activities. You can even play logic games like Battleship.