I'm knitting a summer dress right now, oddly enough, that's pretty, light, and made from a cotton yarn.
The only problem I'm actually having with it right now is finding time to sit down and actually do it. It's a VERY easy pattern (the most complicated part is the lace on the bottom and top, and all you need to know for that is K2tog and yo), and it should be a good summer dress. Though, perhaps, with a different, less see-through top pattern (I'm just going to wear a slip under mine, I don't care if lace or straps show).
Hope that helped.