DPMS makes (or used to, at any rate) a left-handed upper for the AR-15. Other than that, I don't know of any specifically left-handed semi-autos.
I'm a lefty and haven't had a problem with "normal" semi-rifles or handguns. The AR is probably most lefty-friendly -- easily available, drop-in ambi parts (selector, charging handle). The guns with their charging handles on the left are pretty easy to operate, too (FAL, Robinson M96, et al).
Hope that helps...