If you have updates for this list, PM  Paul Wilder or the mods.

Mask Suppliers

Antrobusandgrimm   - Leather Masks, some stunning commision work.
Evenlodestudio - Excellent Latex 'half face' masks, covering most of the Maelstrom races.
Firecat Masquerade - Rather lovely bespoke masks, mostly Domino-style.
Halloweendirect - Full face masks and make up suppliers. American.
Kangena - Home of The classic 'Big eared Gnoll' mask amongst many others.
Monster Boutique - Mostly known for their masks, also do make-up supplies
NorthFur - American, and mostly aimed at the animal costume market
Pans Devil Horns - Just does Horns. American
Scream Team - Aka Mostly Dead. Excellent undead/fallen resource. American
Rubber Gorilla - Mostly monstous masks.
Fatkraken also does excellent masks (examples 1, 2 , 3 , 4  ) and can be contacted via rohare at hotmail dot com .

Make Up Suppliers

1st Night - Includes the Showmake Up "How to" guide.
Dauphines - A comprehensive range of high quality products
Face Paint UK - A good site aimed mostly at face painters.
Graftobian Make-Up - Brand Name supplies. American.
Stage Makeup Online - Carries the Ben Nye range. American.
WolfeFX - Contains hints and tips. American.

Costume Suppliers

Adventurersmart  - Does a bit of everything
Alhannah - Islamic Clothing
Benloder  - Gorgeous, Bespoke leather armour
Croftdesign  - A religious clothing suppliers
Chows Emporium - Costume and accesories
Drachenscmiede German website. Translation can be found here.
Darkbladeuk - leather armour, mostly
Garbtheworld - Historic clothing, including ancient Rome, Egypt, Greece. An American site.
Get Dressed for Battle - Quality clothing as well as re-enactor kit.
Jas-townsend  - Re-enactment quality kit
Kmgarlic-shoemaker - Re-enactment shoes
Leprevo - Leather Supplier
LRP Store - A one stop shop for all your LRP kit
Primal Forge - Also sell a bit of everything
Velvet Glove - A very wide range of costume available.
Warriors Wardrobe  - Specialises in bespoke kit for classical periods
Jez Hunt also does high quality leatherwork at good prices, his portfolio can be found here

Mask and Make Up Resources

Here Be Dragons - Mould making guide among other top-tips
Facepainting tips - Does what it says on the tin.
Monster Makers - Includes links to video clips, and also a supplier.

Costume Resources

Aldebaran - The Grand Daddy of LRP resource sites. Has prop and kit guides as well.
Alleycatscratch - Costume research with a great "Making 'Em" section
Habithat  - All sorts of Patterns
Costumes - Many Free Patterns
Entropyhouse.com/penwiper - A how-to on foam armour
Lafnmoon - Mostly Western, Civil War, Edwardian and Victorian patterns.
LRP Costume UK - A Live Journal Community, and an old friend to Maelstrom.
Pirates of The  Caribbean Costume Forum - 'Nuff Said
Reconstructing History - Expansive Historical Costume Resource
Sempstress - Excellent Period Costuming Resource

Make-Up and Mask Tip threads on Rule 7

(They are others, but these should get you started).

Make Up tips and Ideas Avians  
Make Up tips and Ideas Dracoscions 
Make Up tips and Ideas Dragons
Make Up tips and Ideas Mokosh 
Make Up tips and Ideas Wemics  
Make Up tips and Ideas Myrmidon 
Easy Myrmidons
Make Up tips and Ideas Ophidians
Make Up tips and Ideas Dryads 
Make Up tips and Ideas Eidolons 
Make Up tips and Ideas Facets 
Make Up tips and Ideas Golems  
Make Up tips and Ideas Undead