singers similar to Lucinda Williams

I'm not sure if you mean someone who sounds like Lucinda, or artists in the same vein. No one sounds like her. She is truly one of a kind. But, artists who are similar in genre you might consider:

Steve Earle
Ryan Adams
Alejandro Escovedo
Cowboy Junkies (Margo Timmins and her bros.)
Rosanne Cash
Gillian Welch

. . . to name a few.

(mofi makes a great suggestion regarding a lw pandora station). i'd put kathleen edwarrds as her closest acolyte (and very near her equal). gillian welch and eilen jewel are also in the same neighborhood.

Lucinda is pretty special, and nothing directly compares, but some excellent people who are kindred "alt.country" spirits are:

Shannon McNally
Julie Miller
Sera Cahone
Kasey Chambers
Amelia Curran
Caroline Herring
Cowboy Junkies
Kendall Carson
Gillian Welch
Emmylou Harris

All of these are excellent. Curran my current favorite. McNally may be closest to Williams, as she rocks the most.

John