You can teach a student a lesson for a day; but if you can teach him to learn by creating curiosity, he will continue the learning process as long as he lives. ~Clay P. Bedford
Programming language.
http://www.w3schools.com/ has allot of programming languages you can learn such as, HTML/HTML5 CSS/CSS3, including PHP, javaScript, VBscript, xml, and many more.
http://ss64.com/ is a website where you can learn how to create batch-script.
Terminal Commands (Oracle Database, Windows CMD, BASH(LINUX), VBScript, OS X commands, Windows PowerShell), SQL Server database
http://www.lynda.com/ is a website that they have a lot of video tutorials including HTML, XHTML, PHP, CSS, C#, picasa photoshop and MANY MANY MANY MORE
Math
khanacademie.nl(NL!) is a math learning website, it has a variety of math related topics, which u can learn. It also has a bunch of videos u can learn from.
khanacademy.org has a bit more stuff to learn not just math, but also Science & Economics, Computer Science, Humanities, Test Prep, and with over 4,000 videos u are for sure to learn something here.
Guitar
justinguitar.com is a, you guessed it, a guitar learning site. If you want to learn how to play the guitar, this is one of the best website out there. Can't wait to start playing Metallica - Nothing Else Matters, try the beginners course, before thinking u can enter the hall of rockdome.
Learn Everything
memrise.com Its not just a learn-a-language website, but one with a much more wide categories, it offers a variate of courses:
- Languages
- Arts & Literature
- Maths & Science
- The Natural World
- History & Geography
- Professional & Careers
- Entertainment
- Trivia
I started 'hacking French' as the website names it, and will continue to do so for a couple of month.