Deus Ex would be a good introduction since while it carries some RPG traits (item acquisition, character development, dialogue choices, multiple quests/routes, differring outcomes) it looks and feels like a fairly standard FPS most of the time. The RPG-elements are not overbearing, it's not like some games where you are swamped with options and stats, never quite sure what to do.
System Shock 2 is in a similar style, but is a bit harder to get into, the interface isn't as slick and in general Deus Ex is miles ahead in my eyes.
In fact one could argue that despite being over 6 years old, DX is yet to be bettered in terms of FPS-RPG hybrids.