I've found some great patterns for market bags. I found them on Ravelry. But I will link to the patterns here for those who (for whatever reason aren't members of Ravelry.
Sully Market Bag
Sundance Market Bag
Recycled Market Bag <--- Made out of plastic bags String Bag
Although, the one I like the best is the Girlfriend Market Bag, but you can only find it on Ravelry.