Over the years, I've found some resources that I think are great and others that are not so great. I figure this is a good post to compile that list to help anyone looking for resources to adopt or avoid.
Paul's Online Math Notes - Easily the best site I've ever seen made. Created by Paul Dawkins at Lamar University, he follows the right formula. Every page starts with "what is the point?", followed by how to solve it, and finishing with simple concrete examples. His examples are fantastic too. They cover the vast majority of permutations you'll see in a homework or testing problem that covers that topic. This is definitely why I got A's on my Calculus classes.
Tutorials Point - This site has great tutorials for programming languages, concepts, and technologies. They teach by example, which makes learning immensely easier and quicker. This company is also constantly creating and trying new methods and tools. Some great and other not so great, I still love how much they offer. You'll find everything from sandboxed online terminals to video conferencing tools on there site.
Stat Trek - A pretty good site for learning statistics. They're explanations are bit cryptic, but they do a good job of showing you how to solve something. Some other draw backs are that it can be difficult to navigate the site (you can end up on their tool for solving a distribution as opposed to the page that shows you how to do it yourself), and they use text for they're math equations, instead of images or MathML.
Brilliant - This site hosts their own interactive online textbooks. It's not free, but their lessons do a good job of explaining things simply. It dives too quickly into asking you questions (they're fully focused on 100% active learning), but that's way better than the opposite of 100% passive learning (*cough* *cough* MOOCs!).
Hackr - On this site, users post links to open books and tutorials that teach programming languages and tools. Then other users upvote the ones they like so users see the best at the top. I think it's a great quick solution for finding resources, but not a long term solution. I hope they expand it to other topics in the future.
OpenStax - Created by professors at Rice University, this site provides free textbooks. While I think traditional textbooks are on their way out, they're still a great free option for classes that still use textbooks or for someone looking for an alternate explanation on a topic.
Wikibooks - They're community sourced books using the Wiki software, which means the layout isn't great, but it's still a good place for free text book type materials.
Slader - Slader provides community sourced free solutions to homework problems in a lot of textbooks. It's a good place to start if you're stuck on a specific problem from a textbook. Solutions are voted up by the community, so you get a better idea if the solution provided is correct.
Chegg - Chegg also provides solutions to homework problems in a lot of textbooks. It's not free, but they have more solutions than Slader sometimes.
Questions & Answers
Wolfram Alpha - This is the big winner in this category. This amazing tool can do a lot, but it's best for math. Want to check your work and see if you got the right answer to a problem? Type in your math problem and this tool will not only tell you the answer, it will also show you the steps!
Stack Exchange - They've done a pretty incredible job of the whole "ask a question and get an answer" type of site by gamifying the process. It's mostly good for simple questions that you want an answer for without pouring through pages of documentation. Some draw backs are that the community can get nasty at times. People tend to forget that sometimes you're asking a question because you're confused, so obviously the question will need refining after a few comments to clarify things. Also, since answering all the easy questions get taken up first, it's difficult for new people to gain points on some of their sites where only complicated questions are being asked now.
There are tons of vocational sites. Mainly because they're easier to make and are targeting an audience looking to expand job skills, so making quick money is easier. So, I'm just going to list my favorite site, but I'm sure there a lot of other good ones (udacity, udemy, etc.).
Lynda - It's a paid MOOC service. While I'm not a fan of MOOCs, I think they work better for vocational knowledge, since usually you already understand (or should understand) fundamental concepts and the videos are just to explain some new technology. For example, I use the site when I want a crash course on a new programming language. I ignore the parts of the videos where they teach how to program, something I already know how to do, and just pay attention to where they show you how a language does things in their own way.
Not So Good Sites or Sites I'm Not Familiar With
Pretty much all of these sites are going to be MOOCs (Massive Open Online Courses). You'll see some pretty popular ones listed below, but just because they're popular doesn't mean they're actually good at educating. The central issue to MOOCs is that they're like the History Channel. They can be entertaining to watch at times and you feel like your learning by just sitting and listening. But in reality, you're not really learning anything if you can't put it into practice. Pretty much all MOOCs fall into that category. Some of these sites try to fix that isse by providing homework or tests, except that you don't have to do them, and unfortunately, anytime you make something optional, students rarely do it.
Khan Academy - I'll admit that Salman Khan is a good teacher. That's why his youtube videos exploded. The thing that kind of scares me though is that he wants to be the only speaker in all of the videos, and his goal is to get everyone in the world using his lessons with the idea that they get their passive learning (lectures) from him and their active learning in the classroom. Can you imagine everyone learning from the same person?! This site definitely breaks one of my core principles in education that diversity is the only way to successfully ensure that everyone finds a resource they can learn from.
MOOCs + University Homework/Testing
Coursera - They offer university classes online. I believe they're all free. These sites have some pretty big issues though, such as creating a university's community, cheating prevention, and learning diversity. They appear to tackle the community issue by giving all enrolled students online chat tools (not a great replacement, but it's something), and they're using Keystroke Biometrics to identify users, which is a way to identify users by their typing patterns. Learning diversity is an issue here because as soon as a big name university like Harvard or Stanford posts a class, students will only want to take their class and no one elses. That's just how good some universities have marketed themselves, it has nothing to do with the quality of the education. So this only exacerbates the problem of a stagnate educational system that doesn't update its technology or methodologies. Another big disadvantage: they offer certificates, which probably don't transfer as university credits.
edX - They're basically the same as Coursera and their biggest competitor.