Avengers toys are particularly popular in 2012, given the release of the film, The Avengers. They are mostly popular among boys, but are also suitable for girls. Due to small pieces, most of these toys won't be suitable for very young children.
Avengers action figures retail for about $10 at most major toystores. Each is about 4" tall. They include Iron Man, Thor, Black Widow, Captain America, the Hulk, Hawkeye and Loki. They also come in some variations (Loki with or without cosmic spear, etc.)
Avengers toys are also popular within Lego: