Sites using the Open Source OpenCart e-commerce system, available in many web hosting packages, can configure their stores to use third party payment processing companies. This enables companies using OpenCart to accept bitcoin from their customers with minimal effort or risk.
European OpenCart Compatible Payment Providers
BitcoinPay.com is a European  Bitcoin Payments processor. It has a OpenCart Bitcoin Payments Plugin which registered users can integrate into their e-commerce stores, to enable them to take payments within the Eurozone, as well as local payments in Czech Koruna, and Polish Zloty. Settlements can be set to remit in any percentage of fiat and bitcoin.
Direct to Wallet OpenCart Module
Blockonomics has an opencart module that allow merchants to receive bitcoin directly to wallet. All major HD Wallets like blockchain.info, electrum, trezor are supported. The plugin is open source and can be access on github
Old OpenCart Bitcoin Extension
An older extension made by John Atkinson (jga) of  - the original source of this page - is no longer actively maintained as of mid 2013. Information relating to it is below to maintain the record.
An OpenCart payment module that communicates with a bitcoin client using JSON RPC.
This code accurately converts USD to BTC using the up-to-the-minute MtGox average. It is completely self contained and requires no cron jobs or external hardware other than a properly configured bitcoind server. Every order creates a new bitcoin address for payment and gives it a label corresponding to the order_id of the order. It installs like any other OpenCart plugin and it is completely integrated with OpenCart.
OpenCart Bitcoin was first made available on September 21, 2012.