well, the leaving it on the bike changes my recommendation a little.
I have these
http://robonza.blogspot.com/2007/06/...asket-aka.html
I got them on sale for about $25, looks like they are on sale now
http://www.nashbar.com/bikes/Product...0052_170997_-1

They are nice, simple... They can be stolen, but fold flat when not in use and are unobtrusive so maybe wont be. They are also really really easy to remove, I use them as my shopping bag so I don't need to get "paper or plastic". But, I've recently switched to these
http://www.calhouncycle.com/productc...idproduct=1445
Quite a bit more expensive and huge. I got these to bike camp with when we started riding out after work Friday, camping, and riding home again - nice simple bags for that and they make really nice big carry all grocery bags, too.

But, based on leaving on the bike, the townies are nice, light, simple, functional, and relatively inexpensive.