Adderall (amp) is pretty good, especially XR since it lasts longer and has less of a crash. I sometimes find it easy to get distracted on XRs though, but I think that's just me.
Piracetam is good non-pushy alternative to adderall. I find it helps me concentrate and focus on doing my work. It's nowhere near as "in your face" as amp though. And it doesn't seem to work for some people.