There are approximately sixty million exhaustive mtb links pages out there, so I'm not going to try and replicate them. We're only going to list sites and companies we think make good stuff, are interesting, or give good service. That way we can find the time to give a little info on them too, and give special attention to UK companies, 'cos no-one understands mud like us!
       
Site of the Week or two...    

chainreactioncycles.com
Ideally, your LBS would have everything you needed at the right price. But that doesn't always happen, and when you need to look further afield Chain reaction is a great place to go. It tries to be as close to your local shop as it can be, whilst still having huge stock at some of the best prices around. There's no physical staff and mechanics to talk to, but you can call or e-mail your questions, and better still most items are accompanied by owner reviews. Delivery is very quick, and best of all it's free! A lot closer to the Local Bike Shop experience than most. It's good to support your LBS, but when they can't help, or you haven't got a good one near you, you should check them out.

         
Bikes        
UK  
Curtis wonderfully made dirt and trail bikes from the uk company with a great history
Pashley good site by Britains most versatile bike builder.
Orange Lavish, info packed site about the very impressive new range
24seven very cool, simple site. fantastic bikes.
Saracen a huge range and good value to boot, good site
on-one cracking site with spanky singlespeeds and other gear from the guy that was MBUK's Zak Tempest...
Pace slick and useful site and loads of info on Yorkshire's great innovators
Identiti some very nice burly frames and other stuff
Muddy Fox many people's first intro to mountain biking, still knocking out great bikes
Dawes constantly improving and offer killer value, you'd be mad to ignore them
   
Best of the Rest of the World
Cannondale a fantastic site and some of the best designed, most ridable and innovative bikes out there- good clothing and mx bikes too
Ellsworth relatively new on the uk scene, prepare to drool!
Specialized Great info packed site, still a great bike maker
Kona I miss the cool Hawaiian names but the bikes still rock, read about the 'groove centres', better than the name suggests
Trek a mega-corporation but if they were evil they couldn't make sweet riding terrific value bikes like these...
Gary Fisher One of the granddaddys. interesting site, interesting guy
Klein once utterly exclusive, then bought by trek and cheap-ish, now reassuringly expensive again- Kleins have always rocked
Yeti one of the originals, with a cult following and much less neon paint jobs these days!
Rocky Mountain slightly up-scale but just about affordable and so nice- if you haven't done, check them out
Santa Cruz anoter aspirational brand- do you really need a car..?
Marin a mainstay of the mtb scene, always great and embraced full-sus for stuff other than DH earlier than most
Turner expensive but worth it, cool site
Giant from bargain basement to off-the-scale, I've always had a soft spot for Giant bikes. Don't forget to check them out.
Intense very well reviewed, great looking bikes
Scott Some amazing bikes at great prices, never forget Scott if you're after a new bike
         
Clothing        
UK  
Howies design, quality and ethics make howies stand out, the site is beautiful too
Dirty Habit what other bike-gear company does folding trail-shovels? dh are 100% riders who sell stuff
Karrimor has it's origins in red-sock activities, but has put so much into mtb'ing. the gear is fantastic so make sure you say thanks!
Royal Racing very nice site with lots of Steve Peat wearing the merchandise- enough said!
   
Best of the Rest of the World
Fox the guv'nor. the quality, looks, fit and image are all exceptional. decent site where you can order, too
Scott under-publicised mtb gear- the sunglasses in particular are very cool
Smith the lesser known rival to Oakley, the big O site is better but on the trails it's not so clear cut...
Oakley better than Smiths' but a slightly fiddly site-can't fault the product though
   
Components        
UK  
Hope the best disc brakes anywhere in the world. very useful site
x-lite the daddies of lovely, shiny bike things! very nice site
on-one same cool site, many cool components
planet x just about everything on here, great range and prices and a uk company
Goldtec british made hubs, supension tweaks and other dh stuff. slightly shabby site but wicked products
   
Best of The Rest of The World
SRAM much nicer than the Shimano site, but not as useful.
Shimano not the simplest site but loads of info including product manual downloads (for when yours get too greasy to read!)
Mavic the daddy (papa?) of wheels, the site could be quicker but it's all there
Marzocchi 10/10 for the site and the product
Ritchey the guy has staying power, and plenty of talent. my Ritchey sadlle is indestructible...
Avid great brakes, great site
   
Shopping        
UK  
Chain Reaction good range of components and accessories, great prices and quick, free delivery. Limited clothing range only downside.
Mountain Trax no online ordering but helpful and well informed and great range. Well worth a visit to the actual physical shop
BikePark slightly road-y but good technical kit and loads of bikes.
Evans big range and reasonable prices, good customer service. Astonishingly slow site though...
Edinburgh Bicycle Co-Op good range, prices and info. You can tell they know their stuff.
Wiggle great range and sometimes fantastic offers. Expensive postage so spend over £50 for free delivery or go for a really big bargain!
Stif good looking site with some great bargains and really hot on clothing and higher-end bikes. Only downside is that 'stifsport.com' can get blocked at work!
Stows good deals and service, the shop in Slough is a bit of an alladins cave if you're in the area
BionicSport great new site sellings mtb, bmx and very cool looking kites! Lots of info and easy to use, check it out
Leisure Lakes Not a pretty site but packed with stuff and quick deliveries
Bikedock Online warehouse with a good range and prices
   
General  
UK  
Charlie Kelly the personal site of the great man himself
Steve Peat Great site with Steves diary giving all the goss from the World Cup.
BikeMagic Fantastic resource of reviews, news and all things UK mountain biking. Check out any potential purchase in the Gear section first
RATZ MBC The site of the excellent Reading All Terrain Mountain Bike Club- friendly and have their own amazing track
Singletrack The site of the very cool magazine
Bigfoot MBC Nice club site, if you live anywhere near them you should join!
Plantbase the best plant database for all you mtb'ing gardeners
   
Best of the Rest of the World
Mtb hall of fame a real museum in Crested Butte, USA, the site is fascintating reading. Spot the British HoF entrants!
Wrench Science spec your own bike down to the inner tubes, get the total price and weight and everything! Great fun even if it is in dollars
Bikepro this is an awesome, fact-filled and exhaustive database of products- but eerily it stops at 1997. Have they gone bust? Lost interest? Fascinating anyway, but a really great snapshot of mtb'ing five years ago