In a recent article "Traffic around Mars gets busy" the NASA/JPL summed up some facts about spacecraft currently orbiting Mars. The most interesting is the provided graphic of their orbits around Mars (together with planet's two natural satellites Phobos & Deimos):
So there are 5 active Mars orbiters currently orbiting the planet:
The most basic facts about Mars in a popular science video by Phil Plait, Crash Course. More of Crash Course educational videos about Astronomy you can watch here.
Picture of the Day 25/4/15 - Spaceship taking off from Mars (1990) by space illustrator Pat Rawlings. His description about this drawing you can read here.