John,
I have a couple OTR albums and really like them. Another CDBaby find was Third Road Home - Sister/brother act, and Trinity has a very good voice with better songwriting talents ... al but one cut is hers.
One of my fav's is Tift Merritt - her first sounds like the premature reincarnation of Emmylou Harris, her second is more roots rock, and again she writes almost all her own music. And she's a great kid (well, not a kid anymore actually)
Some other somewhat different non-traditional-best-female vocals:
Ruthie Foster (try 'Hymns' - it's not what it sounds like)
Kasey Chambers (first one more 'country', second and third more rock, but categorization is blurry)
Ollabelle - bluegrass and gospel
And a final favorite - Maura O'Connell. Try 'Stories' - produced by Jerry Douglas (who also plays on it along with Edgar Meyer, James Taylor and many others). More of the 'traditional best voice' with a Celtic/Country/rock twist.
