A Polish letter modifier. (Gritchka mentions it is also used in Lithuanian.) Vowels have ogoneks attached to them to show that they are nasalized.
Typographically, the ogonek is not a cedilla, although it looks similar to one. Instead of being a curve open to the left, it is open to the right, like a "C". It's apparently not supposed to be attached at the middle of the bottom like the cedilla, but is part of the basic shape of the letter.
The word "ogonek" in Polish (literally "small tail") apparently refers to the stem of an apple.