• http://www.kalva.demon.co.uk/
  This site has an excellent collection of problems that have been asked in various Mathematics Olympiad (about 4,000 Olympiad problems and about 900 other problems).

Java

• http://www.gjt.org/
  The Giant Java Tree is an open source project whose primary aim is to create a Java source tree consisting entirely of open source.
• http://www.gnu.org/software/java/java-software.html
  This includes free software from the GNU project and other sources.

Online EE materials

• http://www.epanorama.net
  This is a web site, which is dedicated to offer information on electronics found from the web. This is perhaps, the best web site for electronics.
• http://www.elec.mq.edu.au/~cl/teaching.html
  This contains Lecture notes of Department of Electronics Macquarie University.
• http://scitec.uwichill.edu.bb/cmp/online/online.htm
  This has notes on Compute Science, Electronics and Physics.

Others

• http://www.thinks.com/
  For Games, pastimes & Leisure.
• http://www.howstuffworks.com
  For those who want to know how things around us work.
• http://www.astalavista.com
  For those interested in computers and bit of Hacking.
• http://www.crack-locator.com
  This site has cracks for almost every software.
• http://acm.uva.es/problemset/
  This is a problem set archive for ACM International Collegiate Programming Contest.