A lot has been said about different kinds of math resources here in MO. 
To mention a few:

[Most helpful math resources on the web][1]

[Undergraduate Level Math Books][2]

[Best online math videos?][3]

So having benefited a lot from free resources on the net, I thought it would be more helpful if I shared my list of important sites and learned from others, too.

Here are my list of sites providing free math resources. Some of them are file hosting and sharing sites and others may need registration.

1.[ Math Online][4]  Recently launched by Andrea Ferretti

2.[MIT OpenCourseWare][5]

3.[Project Gutenberg][6]

4.[2020ok  Directory of FREE Online Books and FREE eBooks][7]

5.[Freebookcentre.net ][8]

6.[Gigapedia][9]

7.[Scribd ][10] is a social publishing site, where tens of millions of people share original writings and documents. Scribd's vision is to liberate the written word.

8.[4shared.com - free file sharing and storage][11]

> My question here is to find more free
> resources sites possibly specializing
> in a given branch of math. Please
> specify a direct link anf whether
> registration is required.

 - PS: One technique I use is to google my desired file with its file name and  type as in "Elliptic curves pdf"(which returns a list of pdfs on elliptic curves) or "Random matrices doc" (for word document search) and "Ramanujan history ppt"(which returns a list of presentations or slides).

*You may also suggest such search techniques.*


  [1]: http://mathoverflow.net/questions/2147/most-helpful-math-resources-on-the-web
  [2]: http://mathoverflow.net/questions/761/undergraduate-level-math-books
  [3]: http://mathoverflow.net/questions/1714/best-online-math-videos
  [4]: http://mathonline.andreaferretti.it/
  [5]: http://ocw.mit.edu/index.htm
  [6]: http://www.gutenberg.org/wiki/Main_Page
  [7]: http://2020ok.com/
  [8]: http://www.freebookcentre.net/
  [9]: http://gigapedia.com/
  [10]: http://www.scribd.com/scribid
  [11]: http://www.4shared.com/