Go to an empolyment agency, and find out what the market is offering. Also check out your companies policy on 'pay for performance'.
Then you have 3 options:
[1] Get a new job with another company at the market rate.
[2] Use the information to justify a pay rise [I did this and got $20K in one year] with your current employer, if he up's the salary great, if not go back to options [1].
[3] Do nothing and let it affect your work - the worst option of all.