Is there a website that allows one to trade multiple different cryptocoins (Bitcoins, Namecoins, SolidCoins, RuCoins, LiteCoins, etc) all in one place?
BTC-E currently offers trading pairs of BTC/USD, BTC/RUB, SC/BTC, SC/USD, GG/BTC, TBX/BTC, FBX/BTC, LTC/BTC, LTC/USD, RUC/BTC, RUC/USD, RUC/RUB, NMC/BTC and RUB/USD
Vircurex.com now offers traiding support for many of the crypto currencies: BTC, NMC, LTC, LQC, I0C, IXC, DVC, GG and SC. You can trade between any pair of these at will. USD and EUR are also supported to buy or sell all these crypto-currencies.
There is a pretty comprehensive list over at TradeLi.st
Fiat BTC/USD BTC/RUR BTC/EUR LTC/USD LTC/RUR LTC/EUR NMC/USD NVC/USD USD/RUR EUR/USD Crypto LTC/BTC NMC/BTC NVC/BTC TRC/BTC PPC/BTC FTC/BTC XPM/BTC
Coinex, Virtuex, Cryptsy (too many to list)