Nowadays, it's very easy to satisfy the desire to learn something by logging in to the Internet. While there are a number of paid services, free is the word of the day! Whether you are currently in school or simply interested in a topic, you have your choice from several sources. Here are three of my favorites.

MIT OpenCourseWare
The Massachusetts Institute of Technology offers a large number of courses online for free. A class can include a combination of lecture notes, exams, labs, background reading, and even video. Many of the popular classes are the typical MIT topics such as engineering and computer science, but there are also liberal arts classes available as well as classes from its Sloan business school.

Khan Academy
Khan offers a variety of videos each dedicated to a single educational topic. I've personally found it to be an excellent way to reinforce concepts for children. I tutor students in Algebra, and its a great resource to ensure that I am up to speed on alternate ways to attack a concept. Additionally, it's a great resource for my students to browse between our tutoring sessions. Think of it as YouTube for education. Wired recently had a great story on this resource.

Online Tutoring
If a traditional one on one role is needed, there are several websites that connect people with tutors via text or video chat. They often offer an online whiteboard to be able to work out specific problems with students. Communication is through text chat, with the option for voice chat. These services cover various classes, from science to English to history. The example given, Tutor.com, provides free access in several participating libraries in the US.

Any other cool sites?