Almost anything can serve as a background in After Effects and there's a bunch of background presets built in. Motionworks has some tutorials and free projects, and AE Help discusses how to create a seamless loop using Fractal Noise, as does Adobe's Bob Donlon (but with pictures) in Some Background Information. Laurence Grayson has several textured background quicktips videos; some are noted in Carbon fibre+ brushed metal in After Effects at AE Portal. And here at AEtuts+, Adam Everett Miller shared Quick Tip – Loop Your Textures For A More Dynamic Background.

One resource though seems to specialize in creating backgrounds in After Effects, Church Media Design. CMD offers related free video backgrounds, expressions, AE projects, and other resources along with their tutorials. [For more on looping see Trent Armstrong's site noted  in comments.]  Here's a selection of their shows that offer tutorials on backgrounds (newest first):

• Pattern Backgrounds Inside After Effects
• Looping Backgrounds: Wiggle Effect
• Looping Backgrounds: Particle Effect
• Looping Backgrounds: Fractal Effect
• Looping Backgrounds: Offset Effect
• Vintage Film Grain
• After Effects Kaleidoscope
• Spiral Sunrise

Here's some example shows from CMD: