The following are all about model making etc. rather than the forum side of Warhammer. These will hopefully provide some useful extras for conversions or basing.
http://www.evilmushroomgames.com/resin_bases1.htmEvil Mushroom has an excellent supply of resin bases at an excellent price. I found their service okay, but it was over a bank holiday. Very cheap and good quality.
http://bitzbarn.com/A well stocked Bitz store for all your conversion needs, I am aware of others but this seems the best stocked. The service was slow, they didn't answer their emails and postage is expensive, coming from the states (obviously not so expensive if you live there!)
http://www.forgecraftgames.com/catalog/Again more, varied bases. Haven't ordered off this site so I can't comment on the quality/service. But it seems okay.
http://www.dragonforge.com/good but limited range. Nice bases but not enough of them. Again I haven't ordered off this site so I can't comment on the quality/service. But it seems neat.
*edit It transpires if you look further into the website they actually have a HUGE range of bases, so scrub that bit about a limited range. Also their shipping costs are really great too.
http://www.back2base-ix.com/good for your gaming table with a series of uniformly sized segments that neatly slot together. Hard to navigate.
http://www.microartstudio.com/Only the bases of interest for 40k players but some other interesting stuff for fantasy players, a bit expensive but a good range
http://www.kerrandking.co.uk/Some good urban scenery otherwise not so great, still might be useful for some imperial city
http://www.ironhalo.net/store/catalog/a good model painting service but also an excellent range of bases, a bit samey compared to other sites but these hopefully useful for making that figure stand out on the battlefield
http://www.darkartminiatures.com/a bit of an O.T.T. website but some good, and expensive, stock. Especially for Chaos and Dark Eldar
http://wargamesfoundry.com/an excellent site for miniatures and slightly alternate scenery. Good range of stock and good quality, but a bit pricey
http://www.bitzbox.co.uk/Another Bitz website, better for UK customers (cheaper and quicker) than Bitz-Barn but not as well stocked
http://www.westwindproductions.co.uk/catalog/index.php?cPath=130west wind productions as a whole do an excellent range, but this links straight to a thing which is best about them. SHS (Separate Head System), basically you buy just the heads! This is brilliant for conversions. I love this website, strongly recommended.
http://www.jrminiatures.net/more focused on historical figures, but some good scenery. Not as high quality as some other sites but different.
http://www.miniaturescenery.com/Miniature scenery produces novel slot together scenery. Some good stuff, but it isn't the scenery I am so keen on. They produce these excellent desk organisers, again slot together, which are perfect for the hobby bench, you'll see what I mean when you look at the store
http://www.worldworksgames.com/store/Haven't ordered from this store because I have found it a look cardboard looking. However for large scenery projects where scale over quality is the aim, it might be useful.
http://www.fantascene.net/cart/Fanta-Scene sells good bases and some smaller pieces of scenery, good for objective markers. I haven't tested their service so no advice there
http://www.daemonscape.com/index.htmlNot the greatest models, but again good bases, and some novel ideas for scenery. A bit expensive, I'm afraid I haven't tested their service
http://www.wargameterrain.co.uk/Again, more focused on the historical side of the scenery but some useful cross-over. Very good prices
http://www.zuzzy.com/Zuzzy concentrates more on roll-out mats than physical scenery, but this seems high-quality and fairly priced. I haven't got or made a gaming table so no need for their products
http://www.wargamesterrain.co.uk/page2.htmlWargames Terrain has some good general natural scenery i.e. rocks. A bit expensive but they appear very high-quality. Unused from me unfortunately due to their price but they appear to offer a very good service especially in quality
That's it for now. This is my first ATT post so I hope it is in the right section and everything.

. Hope this is useful
*Edithttp://chapterhousestudios.com/webshop/This website directly endorses Games Workshop not so much for us Tau lovers there but some great Space Marine goodies, also some female Imperial Guard heads, they could be converted. Looks good
http://sciborminiatures.com/en_,index.phpMore for the Fantasy player, but you can find some good stuff for some conversions there. Looking at the pictures some REALLY high quality goods at a price though.