Two canonical online references are:

* The Digigal Library of Mathematical Functions ([http://dlmf.nist.gov/][1]) <br/>
This is official successor of the venerable Handbook of Mathematical Functions by Abramowitz and Stegun.

* The Wolfram Functions Site ([http://functions.wolfram.com/][2]) <br/>
An expansive collection of identities and properties of special functions amassed and neatly categorized by Wolfram Research.


  [1]: http://dlmf.nist.gov/
  [2]: http://functions.wolfram.com/