My favorite bag is from a small company out of Pocatello, ID. Krieg.
Sam Krieg started 15 years ago sewing chalk bags for climbing. He made the bags out of wild and funky patterns to bring some personality to a standard and drab product. When he started making bike bags, he continued the same trend.
Krieg saddle bags are just the right size for me. I can stash a tube, multi-tool, CO2 cartridge, and tire lever inside, and it tucks up nicely under my seat. the Velcro strap holds it securely to the rails without the need to use a strap around the seatpost. I have one for both my road and cross bikes, I need to get one for the wife's bike.
You want a fuzzy pink saddle bag? Leopard print? Comics? He can do it.
The bags will run you $22.50. You can find the bags in numerous bike shops around the country, or order them online (free shipping!).
What are you waiting for? Add some fun to your bike. As their motto says: Dream in Color!