Medieval and Fantasy Costuming is one woman's list of sites that she finds useful.
The Elizabethan Costuming Page is an utterly fabulous resource. It's geared a little bit more toward construction (ask me about my corset!), but it is still useful for determining how Elizabethan women's clothing goes together, and what pieces comprise an outfit. Be sure to look at the Elizabethan Costume Gallery for a wealth of useful portraits.
The History of Costumeis a Victorian picture book; the style of the illustrations should be immediately recognizable. It is not very useful as a resource for building patterns or designing clothes, but it should give you a basic idea of what a particular period looks like. (N.B. - When they say "history of costume," they really mean it. It aims to cover pretty much everything that's been worn, ever, anywhere.)
Historical Costuming FAQ The List of Frequently Asked Questions about historical costuming, culled from an Internet discussion group. There are no pictures here, but there is a fair amount of raw information.
How to Wrap and Wear a Great Kilt If you have five yards of wool hanging about the house, and are dying to wear it but you don't know how, here is your answer. Hours of fun with an assistant.
Milieux: The Costume Site is a locus of costuming information. It covers every historical period, not just medieval/renaissance, but there are some excellent resources if you want to do some non-European clothing. (Most of the mainstream costuming sources are European, focused especially on French, English and Italian clothing. As always, the Italians are the trendsetters, and the French think they are.) Densely packed, but definitely worth a look.
A Twelfth-Century Lady's Clothing is an example of how to determine costume details from original source material. Using a twelfth-century ivory of the Virgin and Child, the author reconstructs many details of the construction and composition of the clothing of a woman of that period.
The Guide to Elizabethan Costuming is a rough outline for Renaissance Faire workers. Faires are often a little hazy on the actual authenticity of their employees' clothing, but they do strive to present a uniform and clearly described effect. Useful outline.
The Costume Page is another great big pile of information, a place from which to start out.
Houpelandes is a further exploration of that fabulous consummate 14th-century fabric-waster, the houpelande. Suitable for both sexes.
Cotehardies is another collection of images and explanations of a beautiful garment from the mid-14th century, the cotehardie. Also suitable for both sexes.
Period Styles is an image catalog of period representations of clothing - portraits, paintings, and the like. A bit slow to load, but wonderful for getting an idea of colour, shape and style.