Green Lantern: First Flight, already recommended by Sam, is also one of my favorite animated superhero movies. I watch it rather often-----when I feel like watching some light entertainment with lots of colors (I own it on Blu-Ray and it looks quite stunning), and a short duration. It's a short film-----only 77 minutes.

Also, I recently bought Gotham Knight, which is frickin' awesome! It's a series of interconnected vignettes taking place in between the two Christopher Nolan Batman movies. I got it as part of a Blu-Ray box set-----which also includes Batman Begins and The Dark Knight.