Forgive the blatant plug, but I've just come across a superb and simple java-based .abc program, Skink.
Developed by a chap called Wil Maculay, it runs perfectly under Mac OSX (something which Barfly sadly isn't up to yet without repeated crashes); opening, playing, editing and printing abc files.
And it's free!
Available here.
And no, I have no connection with Wil Macaulay - I just liked his software and thought it deserved a plug.