I'm not sure which of these are "seinen," though I think most qualify:
Puella Magi Madoka Magica - don't let the hype fool you; it really is that good
Bakemonogatari - another SHAFT outing; not my favorite but worth a try
House of Five Leaves - excellent character studies about petty criminals in the late Edo period
Level E - retro comedy; weaker in the middle episodes but a strong start and finish
Durarara!! - same mangaka and studio as
Baccano!; didn't grab me the same way, but it's another one worth trying
Current offerings
Usagi Drop - one of the best mature slice-of-life stories I've seen
Hyouge Mono - power, honor, aesthetics and tea in the "warring states" period
Worth trying
Moshidora - a high-school girl uses Peter Drucker's
Management to manage a baseball team; based on one of the all-time best-selling novels in Japanese history
How about a josei?
Kuragehime