I hadn't realised Harmony Assistant (and poss M A) supported tab. A good thing with both of those and my suggestion is that they do offer abc support.

For W.E., one of the other great things about abc is simply the volume of tunes on the Internet written in this format. Just have a look through the abc collections section from the link Mark gave. It is not as strong on songs (with words attached and aligned) but there are certainly sites, including ours that are trying in this area. I suspect that in time Mudcat/dt may get out of it's MIDI/songwrite thinking and there already is a version of the dt converted to abc. Other formats such as Lilypond may gain wider support but if simplicity is what's needed (we in the folk world are not looking to produce orchestral scores), I don't think there is anything to beat the potential of abc.

Jon