Here is a tiny amount of links related to costuming, the 18th century and other interesting things. I'll hopefully keep updating here as time goes on.
The Costumer's Guide to Movie Costumes - although not kept up to date now, this is the place to go for movie costume pictures and analysis.
American Duchess Shoes - just go here and drool over all the gorgeous shoes
Your Wardrobe Unlock'd - A costumer's companion. There's a monthly subscription fee, but there's a handful of useful free articles too.
Foundations Revealed - The same as Your Wardrobe Unlock'd, but for corsetry
Needle 'N Thread - My go-to place for anything embroidery
18th Century Notebook - A great collection of extant garments
Fashions from History - A Tumblr blog with extant garments, jewelry, art, furniture and textiles from throughout history
Marquise - In depth, historically accurate tutorials and patterns
American Duchess - Tutorials, dress diaries and general awesomeness.
Before the Automobile - When I grow up, I want to be Merja.
Bridges on the Body - Simply inspiring corsetry
The Costumer's Closet - I love Loren's weekly roundups of interesting costume links!
The Couture Courtesan - Historical re-enactment and gorgeous outfits.
Couture Mayah - Amazingly beautiful creations and in-depth dress diaries.
A Damsel in this Dress - The owner of Prior Attire, great articles, tutorials and costumey goodness.
A Dedicated Follower of Fashion - Mainly 18th and 19th century outfits
The Dreamstress - Hostess of the Historical Sew Fortnightly, and amazingly knowledgeable. Also, a fellow New Zealander!
Dressed in Time - Various wonderful 18th century garments, and the occasional horse.
Demode - A wealth of tutorials and research articles, mainly 18th century related.
Historical Sewing - Focuses on the 19th Century, but wonderful general articles and advice
Hyaline Prosiac - More down to earth outfits from an American Revolution re-enactor
Jenny La Fleur - All eras of costuming with great hair and makeup to match.
Koshka the Cat - The blogger who introduced me to historical costuming when I was about fourteen.
A Lass of Yesteryear - Not only period stuff, but some wonderful vintage outfits.
Les choses delecates - Another French blog with gorgeous dresses and event pictures
Maggie's Costume Wardrobe - The blog of the lady behind the Costumer's Guide
Mode de Lis - 16th century to vintage and lots of things in between
Musings of a Couturire - Dress diaries and historical musings
Rococo Atelier - Excellent tutorials and inspiring costumes.
Temps d'elegance - A French/English blog with wonderful costumed event photos.
Frock Flicks - a wonderful podcast about costumes in movies, and the many failings therein.
Historical Princesses - Disney Princesses redrawn with period accurate dresses.
The Duchess of Devonshire's Gossip Blog - A blog dedicated to gossip about 18th century dresses, people and art. You know, the important things.
Marie Antoinette's Gossip Guide - A sister blog to The Duchess of Devonshire, with a focus on France instead of England.
Demode's Costume in Cinema - An exhaustive list of historical movie reviews, organized by century.