after doing the last simplymaya challenge I can heartily recommend the lowman 1.2 rig to play about with learning animations.
Its' got a load of controls (especially in IK mode) and is great fun to use:
As for your own custom rigs, I've tried to make a couple and they haven't been very good so I'd recommend finding some free online tutorials and then progress and possibly buy some tutorial videos from here and the other maya sites. The more methods you learn the easier it will become and the more tricks you'll learn on how to make them work well.
For walk cycles, here's a couple of great little examples:
Good luck and have fun learning about it - I could easily become addicted to it now and cant wait for next year when I've got all my time to put into animating