Ruby
bitcoin-ruby: Bitcoin utils and protocol in ruby
buttercoin: Opensource Bitcoin Exchange Software
bitcoin-client: Ruby library to the complete Bitcoin JSON-RPC API.
bitcoin-central: Bitcoin trading platform made in Ruby On Rails. (Documentation)
money-tree: Ruby implementation of Bitcoin Hierachical Deterministic Wallets
blockchain api: Easy to use interface for all bitcoin related tasks
bps: Bitcoin payment processor to accept payment in bitcoins (stale repo)
rbtc-arbitrage: Automating arbitrage beteween MtGox and BitStamp Exchanges
coinpost: Ruby-On-Rails-based Bitcoin Item listing and marketplace system
bitcoin-exachange: Book on Bitcoin Exchange Implementation in Ruby On Rails
ruby-api: Ruby implementation of talking to bitcoind using HTTP JSON-RPC