EducationalI'll keep this list relatively short.
By now wiki needs no introduction but reminding people about some of its subsites can be useful.
WikipediaIt's sad that google answers no longer exists ="/.
Anytime you're looking for interactive help search for forums of the subject your looking for on google. There are millions of willing and helpful people that can help you. If you make the people feel you're coming off as just saying, "TEACH ME EVERYTHING", or "DO MY HOMEWORK, URRGENT ITS DUE TOMORROW", you won't be helped. I know it seems obvious but too many people do this, especially in Yahoo answers.
Not all sites use ugly notation and weird java applets to explain math problems. You can get help from friends and have fun at the same time using instant messengers that have "whiteboard" features like yahoo messenger and msn messenger. There's even Ajax Based Whiteboards which require only javascript. Search video sites like youtube and google video. Also search for animations on google. For example:
Algebra Flash Google SearchThe same applies for higher level subjects like calculus. In general I recommend the site Webmath.com. Their 100 solved integral problems helped me a lot for AP calc in high school. There's also a promising site called "mathcasts" that uses audio and whiteboard displays to explain various topics.
