AlbumArt on AppSwitcher [Jailbreak]
AlbumArt on AppSwitcher does exactly what the title says - adds album art to your app switcher. I personally like having this functionality because I have a tendency to forget what songs come from what albums. This way I don't have to jump into my iPod app, I can simply bring up my app switcher and look without having to stop what I'm doing.
Once installed from Cydia there's nothing to configure or set up, it should just work. It's available for free and requires iOS 5.0 or higher.
Free - Cydia Search Link