Modcloth: The largest, and most adorable collection of retro dresses (like the one pictured to the right). I love this website because they have styling tips with every product description, and the brand has a strong online community.
Anthropologie: Vintage and indie inspired designs. You can also have a styling consultation on what to pair your dress with if you haven't a clue which shoes and accessories to add. In the product reviews, girls can add their body type, which can give you a better idea of how things might fit.
ASOS: Cute, feminine, sexy and just about any other adjective you can think of to describe dresses. This e-tailer has over 2,000 styles of dresses. Amazing. And each product has a runway clip, so you can see the dresses in motion.
Miss Selfridge: Another shop with an unbeatable range of dresses. They have a designated "dress shop" with categories from day to night dresses to bodycon dresses. And they have a side view of their dresses, instead of the standard front/back/close up, which is very useful.
Romwe: City street style meets online fashion. You sort of have to sift through the dresses, but there are some true gems if you look closely. Plus, everything is pretty affordable and there's free regular shipping worldwide.
Rare London: Fun and funky, I love the dresses on this site. My favorites are the ballerina dresses and strapless styles with fringe. Lots of unique options.
Alice and Olivia: Ultra girly, candy-colored frocks. This collection of dresses screams summer. What's also great is that you can read a fairly detailed description of the dress to get a better idea of what you're buying. For example: "sequins and beading featured on a tulle overlay".
To spoil yourself or for window shopping
Frockaholics: Elegant designer dresses. I'm not a fan of their daytime dresses, but most of their evening dresses are incredible... sweeping floor length gowns, decorated at the bust, or dresses with all over embellishment. Makes me want to host a black tie event at my flat.
Farfetch: Collection of dresses from top designers. Nearly every dress on this site is wishlist worthy. And, they have a fantastic search feature to let the buyer find pieces quicker.
If I'm missing any great shops on my list, let me know!