If you are interested in standard tuning (for guitar) accompanying chords for fiddle tunes, I would recommend Robin Williamson's fiddle tune book (published by Oak Publications originally, but probably still available somewhere). I agree that you can use different chords to great effect, depending on what you want the tune to sound like. I'm not sure if any of the Williamson book is online.

I primarily accompany tunes in DADGAD. But, that said, in 1982 I saw a concert with Dave Swarbrick and Simon Nicol and was really impressed by Nicol's backing of Swarbrick. He was using standard tuning (he did dropped D for a couple of his solos). His use of relative minor chords, and some quick changes were effective. Nicol's role was playing backing guitar, but watching him, you realized he was doing some amazing stuff, without getting in the way of the tunes.