I gradually find that the following things are good for you to choose if you have many choices:

  1. R for Statistics
  2. Python for scientific computing
  3. Gimp for graphing
  4. Tex for typing
  5. WordPress  for blogging
  6. Gmail, google+, google sites, google reader,……
  7. Mendeley for managing your papers
  8. Delicious for discovering and collecting the web resources

All of them have a characteristic, i.e. Open Source or Free. And they all have big communities:

  1. R Revolution;  R bloggers
  2. Scipy.org
  3. Gimptalk
  4. Tex Users Group
And another thing is the following useful website I want to share: