There is midiscan and smartscore (www.musitek.com) and there's an inexpensive one named Sharpeye (I think). There was an earlier thread about this with some great feedback. Try search for midiscan or sharpeye in the forum search - should turn it up (hopefully!)
None of them perform perfectly - there's usually a lot of fixing up to do afterward, depending on what the quality of the page scanned in was.
Keep in mind if you scan something (American) that is post 1927 without the publisher or author's permission and put it on the web you are violating their copyright.