Jack Campin has three online ABC converters listed on his Web page. Two of the three work for me. Here are the links:

• http://www.colinhume.com/Music.aspx
• http://mandolintab.net/abcconverter.php
• http://www.clivew.com/abc.php

For all of them, you just paste some ABC into a box and it'll give you both staff notation and a playable MIDI file. (When using your browser as an ABC application in this way, set it to display the ABC with a fixed width font like Courier).