There are plenty of good modern flute concertos. Two I know are the Rouse and the Penderecki. The Rouse is awesome - a bit depressing though, and the Penderecki is a bit dull at times but still good. I've heard that Nielsen's flute concerto is very good as well. I'm not sure about repetoire for flute alone or flute with piano.
Maybe try some of the pieces listed on these pages:
Flute concerto - Wikipedia, the free encyclopedia
Flute repertory - Wikipedia, the free encyclopedia