As I've been playing through Deus Ex, I've seen my fair share of messages pop up, informing me that I have earned XP, but because there's no explanation for the message, It's not always obvious what I've done to earn the XP in the first place.
For instance, it took me a while to realize I was getting the "Marksman" bonus whenever I shot someone in the head, instead of tranq-ing them in the torso. Some others I've seen a lot, but still don't know what caused them -- "Traveler", and "Expedient", for instance.
What actions (or suite of actions) in DE:HR result in an XP Bonus?