Magic the Gathering

MtgCards.be - Singles cards available. Based in Belgium.
http://www.mtgcards.be

OshkoshMagic.com - Cards and accessories including singles, common card lots, mystery packs, and dice.
http://www.oshkoshmagic.com

MTGSale.com - Source for singles, boxes and accessories.
http://www.mtgsale.com

CCG Bot - Singles and supplies shipped worldwide from the US.
http://www.ccgbot.com/

Troll and Toad - Has a large selection of MtG products for sale. Many rarer cards available such as Alpha, Beta, and Summer Edgar.
http://www.trollandtoad.com

The Wagical Place - Specializes in singles for all in-print editions introduced after Fallen Empires.
http://www.wagicalplace.com/

Bazaar Games - Sales of singles, boosters and accessories. Editions from Alpha to present day. UK based.
http://www.bazaargames.co.uk/

Star City Games - All editions available. Also features strategy guides.
http://www.starcitygames.com/

Omega Cards - Individuals selling cards from most sets. US based.
http://www.omegacards.com/

Brainrod - Single cards, specialises in standard-legal cards.
http://www.brainrod.com

CCG-Warehouse - Offering singles as well as sets.
http://www.ccgwarehouse.com

Cutting Edge Cards - Singles and sealed products.
http://www.cutting-edge-cards.com/

MTGFanatic.com - Offers purchasing as well as selling of cards.
http://www.mtgfanatic.com/

J.D. Trading Company - An internet storefront specializing in singles.
http://jdtradingcompany.com

MTG Mint Card - Singles, decks, accessories and boosters available. Hong Kong based.
http://www.mtgmintcard.com

Smedlock.com - Offers singles, bulk lots, accessories and other related items.
http://www.smedlock.com

Card Kingdom - Singles, decks, and boosters. Also has hints on deck building.
http://www.cardkingdom.com/

Xtreme Games Collectible Gaming Store - Sells singles. Physical store in Illinois, USA.
http://www.xtremegamesonline.com

Mox Diamond - Offering single card sales from all editions, as well as resources such as a card search engine, deck library, and card combos.
http://www.moxdiamond.com

MightyMazs Magic the Gathering Shop - Singles from many editions. Based in Germany.
http://www.mightymaz.de/

Wizards Cupboard - Features a variety of MTG singles, booster packs and boxes. Includes articles and gift vouchers.
http://www.wizardscupboard.com/

Alpha Beta Unlimited - Sales of singles, boosters, and sleeves. All editions available.
http://www.abugames.com/

Magic Inferno - American based online store.
http://www.magicinferno.com