Note that both NGC and PCGS have registry programs where you can list your coins and display images. Actually not a bad idea to keep images off site of your collection. . Both also have forums to join and interact with other collectors, although the PCGS forum is much more active.

NGC - One of the top two TPG's
PCGS - One of the top two TPG's

ANA - American Numismatic Association, well worth joining.
BNS - British Numismatic Society, a fine organization, as with many of the British sites, both dealers and others, the Brits remain well entrenched in the 19th Century. However it's worth joining as they have access to some wonderful information.
ANS - American Numismatic Society, a wonderful organization located in New York and with a treasure trove of information and extensive collections you can visit.

Information Sites

My Collecting - This is my other web site showing other world coins I collect. 
Petition Crown - A collectors site with wonderfully rare pieces and quite good stories about some of the coins.
Coins GB - good collectors site and forum.
Coin Imaging - A site by Mark Goodman with several good articles on photography. His book on photography is well worth the price, even though my skills suck, they are much improved since reading this volume.
Wildwinds - An excellent site for information on ancient coins and values.
Wybrit - a collector who can often be found on the PCGS forum, and has a lot of useful information for collectors of British coins on his site.

Dealers & Auctioneers

Northeast Numismatics - Well known dealers with some exceptional U.S. and world inventory. Many of my British and German pieces come from NEN. Number 1 in my view for World coin dealers in the US.
Dix-Noonan-Webb - A long established auction house in the UK, often some very nice pieces in their sales.
Goldberg Auctions - One of the top auction houses in the US for World coins.
R.P. Coins - UK dealer with a very wide selection, Rob is very knowledgable on early copper in particular, one of the few dealers I buy raw coins from.
Colin Cooke - Another well known UK based dealer, another of the few I buy raw coins from. Good images and honest descriptions when you call or email them.
David Lawrence Rare Coins - Well known dealer and auction house, I've bought quite a lot of early US Commems from them.
James Hanna - Affiliated with Northwest Coins, very pleasant to deal with, attractive coins with good prices. I've picked up some nice German material from Jim as well as the MS67 French Franc.
Heritage - About the largest auction house for coins, also art and collectibles auctions.
Aspen Park Rare Coins - Well known dealer with a very nice inventory of world coins.
Classical Numismatic Group - World and ancients, both inventory and regular auctions, these are the guys that got me started in British coins, very good to deal with.
Spink - A leading dealer and auctioneer of British material.
Dix-Noonan-Webb - they run some top notch auctions, several on my rarest pieces have come through them.
Baldwin's - A leading dealer and auctioneer of British material.
Stacks - Well known dealers and auctioneers in the US, always check their US auctions for early British material listed under colonial coins. Two of my finest Geo. III Halfpennies came through that venue.