Wolfram has been in news for being next Google Killer. Well, its not a Google Killer but for sure a killer web application for knowledge based searching. Besides the main knowledge based search functionality, Wolfram also has a dedicated Online Integrator at Wolfram Mathematica section.
It allows you to solve complex integration problems with a click of a button. Just enter your integration string and click the compute online button to see solution in few seconds. Online Integrator at Wolfram is an excellent web service for students and this functionality is also available in download-able version for students (but for a price)!