Owing to a quirk of copyright laws in the 1990s, no works have entered the public domain in the U.S. for twenty years. That ended yesterday, when all works created in 1923 became public domain.
While these works aren’t exactly new, they do offer new material for composers and arrangers to use without the often difficult (or impossible) copyright wrangling.
Comprehensive lists aren’t easy to come by, but here are a couple good places to start for choral creators:
Wikipedia (poetry)
Wikipedia (songs)
I’ll be poring over these pages and other sites in the coming weeks and months, looking for materials to set. I hope you find something, too!