$2.00 Cargo Nets
For those of us without longtail bikes, who just have normal bikes with racks and baskets, cargo nets are great if you want to load a container above the rim or just if you want to keep something from bouncing up/out while riding.
FYI, Walmart is selling cargo nets for 2 bucks. I just picked one up. It's rather small and it's a triangle shape with only 3 hooks, but for 2 bucks you can't really complain. I think I paid 10 bucks for my last cargo net, and I had to special order it because none of the local bike shops had them in stock. That cargo net is bigger and better, but the little 2 dollar guy will no doubt still come in handy.