I'm really late to the thread and I see that Inge and others have already found about a billion possibilities that I can't wait to explore for my own selfish reasons. I will only say that dresses in summer are my salvation -- I far prefer them to shorts for keeping cool and looking pulled together. I can't or don't wear ditsy patterns or strapless or spaghetti strapped or low necked versions, and I don't look good or feel comfortable in minis -- so for years dresses were hard to find and I reverted to shorts during that period. Am glad to have found so many dresses at retail in the past few years.
I really relied on my Bodens last year. The styles are a bit "ladylike" at times, and the prints can tend to be a bit whimsical for my taste, but the quality is superb, which makes up for a lot to me. I thoroughly enjoyed wearing those I bought last year. I'm already wearing them again. What I'd also say about Boden is that if you like the style but not the print, many of their dresses do come in a plain black or navy or sometimes olive. I was already well stocked with solids and wanted to venture out a bit when I ordered these, but it's something to keep in mind. Their service is terrific.
My current summer dresses are below. The first 3 are Boden. 2 are basically "dressier" t-shirts (so very very easy to wear and wash). The gingham is a loose and very comfy shirt dress.
Dress 4 is from Comrags. It can be worn casually and I do wear it as a day dress but it also works for going out. It's the closest thing I have to an actual "sundress" -- the cotton is extremely light and it's fantastic for a very hot day as it is loose and drop waisted. Pricey, but I'll have it a long time. (On its 3rd year now).
The Topshop dress is too warm for super hot weather but great for transitional warm weather, like in May and June and September. It's easy wash and wear and the cut makes it a bit special. I know you might not care for a fitted torso, but my point is more about looking for an interesting cut overall -- it can add to the pleasure you take in the dress.