Hang on to your wallet! here's a bunch of useful FS add on stuff websites:
www.avsim.com (lots of free stuff in the file library, and home of many useful forums plus my reviews and a few things by me in the file library too! Before you buy anything at all, see if there is a review of it on here and check the forums here for any negative comments on potential purchases. Avsim is THE biggest flight simulator website and if there isn't info on something here, then it's probably not out there. It does have a small online shop, but the shop is pretty limited in scope.)
www.flight1.com (lots of add-ons, generally airliners and private planes, mostly compatible with FS9 and FSX. This company also makes simulation software for real-world pilot training, so as you can imagine, their FS stuff is usually pretty astonishing, check out their Beech King Air B200 if you want total realism, if you are looking for something less taxing but still realistic, check out their Cessna 441 Conquest II [you can find a review of this, by me, on avsim's site])
www.precisionmanuals.com (home of the legendary PMDG Boeing 747 and 737 among others, soon to be home to a fully-featured Airbus too. If you can fly one of these, you could probably get the real thing off the ground too. Their support is appallingly bad for download stuff, so avoid buying downloads from them if you anticipate problems and go for the boxed stuff instead, as authorising their online purchases is a right pain in the ass.)
www.alphasim.co.uk (home to lots of inexpensive add-on classic military aircraft. Not big on interactive cockpits, i.e. if you want every switch to work, these often don't feature that, but they are very cheap and great fun)
www.shockwaveproductions.com (home to obsessively detailed WW2 add-on aircraft such as the B17, Mustang, Spitfire and bf109, P40 Warhawk [which yours truly is currently reviewing for Avsim]. If you want every switch working in the cockpit, these are for you)
www.justflight.com (home to add-ons of all kinds, from helicopters, fighters, bombers, Cessnas etc etc to scenery, plus many utilities to make flying more fun, there's lots of FS9 stuff here, and they often have sales on Thursdays if you buy online, so you can save even more cash! Their support isn't as good as other places [not appalling, but not as good as others])
www.flightsim.com (Rival to avsim which claims to be the world's number one flight sim website [it isn't, however it's probably better for news announcements than avsim] and also home to lots of free stuff, however, access to their file library is not as good as avsim's and isn't laid out very well, still well worth a look.)
www.simmarket.com (good place to browse products, they sell allsorts here, everything from books on how to fly an airliner properly to add on utilities. Well worth a look)
www.flyawaysimulation.com (another online shop worth a look at)
www.aeroplaneheaven.com (another online shop, but with the odd freebie add-on aircraft available for download, worth checking out)
www.fsdownload.com (another freeboie download site)
www.abacuspub.com (This site may look terrible, but it has some nice stuff for FS on it and is also home to FS Design Studio, so if you fancy designing your own aeroplanes and you can't be bothered learning GMax or 3DSMax, then this is worth a look.)
www.saitek.com (the good old joystick manufacturer is currently going into overdrive where flight simulator add-ons are concerned, with lots of interesting products in the pipeline, such as throttle quadrants, etc)
Chock