Former / potential future nominees:
cbitcoin - A Bitcoin Library In The C Programming Languagecbitcoin is a bitcoin library written in standard C99. It is currently in alpha-stage and should only be used experimentally. The purpose of the library is to make it easier for developers to create bitcoin applications and the library should be versatile enough to be used in many bitcoin projects. It is not a client library, rather it provides the basic building blocks of the bitcoin protocol.
The features includes:
Bitcoin message structures including serailisation and deserialisation functions.
A bitcoin script interpreter.
Functions for building and verifying merkle trees.
Basic functions which can be used in various validation models (full nodes, pruning nodes, SPV nodes or whatever).
Asynchronous networking code whcih provides a simple interface to the bitcoin network for sending and receiving messages.
A network address manager
Automatic bitcoin handshakes, automatic peer discovery, automatic pings.
Base-58 bitcoin address encoding.
Simple reference counting memory management.
Doxygen documentation and well-documented source code.
Purely standard C99 with weakly linked function prototypes for cryptography, PRNG and network dependencies.
Implementations of the dependencies using libevent and openssl.
http://cbitcoin.com/https://bitcointalksearch.org/topic/cbitcoin-bitcoin-implementation-in-c-currently-in-development-80521OpenPayOpenPay is a new
EMV compatible payment network that rides on top of the bitcoin network, and uses bitcoins as a currency agnostic interchange medium. The end result of OpenPay is that you will be able to spend your bitcoins at any merchant that is able to accept Visa or Mastercard.
For the consumer, OpenPay means you will be able to use an EMV chip & pin smartcard, an NFC capable device such as your smartphone (there will even be a magstripe card option for those of us still in the dark ages) to spend your bitcoins at your local merchant.
For merchants, OpenPay means you will be able to accept payment in your choice of currency (BTC, Local, or other), with no network fees* or fear of charge-backs.
OpenPay is completely de-centralized, the Open Payment Alliance is a volunteer organization tasked solely with the development of the OpenPay reference implementation and standard. OpenPay's design and reference implementation are completely open, and will be released under the terms of the GPL.
https://bitcointalksearch.org/topic/announce-openpay-entering-burn-in-shake-down-alpha-test-phase-92055MultiBitMultiBit is a secure, lightweight, international Bitcoin wallet for Windows, MacOS and Linux. Its primary focus is being fast and easy to use, even for people with no technical knowledge. It has a YouTube channel to help you learn the software, and includes helpful features such as an exchange rate ticker. MultiBit supports many languages such as German, Spanish and Greek.
MultiBit synchronizes with the network much faster than Bitcoin-Qt and should be ready for you to use within a few minutes. This is a good choice for non technical users who want an easy to use experience, especially if you use a Mac.
MultiBit is free and open source
http://multibit.org/BlockViewerThe bitcoin block chain is a virtual accounting ledger which is passed around, appended, and agreed upon by people who are sending numbers to each other.
Blockviewer.com not only attempts to visualize this growing document of debits and credits, but it also attempts to derive more useful and revealing information about it by constructing an ownership network.
It is scraping user's signatures from these forums. If you have a bitcoin address in your signature, your ownership node will be identified. This means that your single signature address can identify potentially thousands of other addresses owned by you (and addresses that might be redeemed by you in the future).
http://blockviewer.comhttps://github.com/thallium205/BitcoinVisualizerArmoryArmory is an open-source, python-based, wallet-managment platform for the Bitcoin network. It runs on top of Bitcoin-QT and brings a suite of high-power features to the average user, including easy offline wallets and an intuitive interface for managing and spending money from the offline system.
http://bitcoinarmory.comHall of FameOpen-Transactions is a solid, easy-to-use, Financial Crypto and Digital Cash LIBRARY, including a working SERVER, API, CLI, and TEST-GUI.
Open-Transactions allows users to issue and manipulate digital assets. Users may create pseudonyms (public keys), issue currencies, and create asset accounts. Users can transfer digital assets securely between accounts (even a server cannot change balances or forge transactions.) Users can also operate "cash-only" (without accounts) for maximum anonymity.
Open-Transactions supports a range of financial instruments, such as cheques, vouchers, and untraceable digital cash. These are all analogous to the same financial instruments that we all use at normal banks today.
Open-Transactions also implements higher-level, contract-based transactions such as payment plans and markets with trades. The markets on Open-Transactions support market orders, limit orders, fill-or-kill orders, day orders, stop orders, and stop limits, just like trading on a real market. OT also supports basket currencies.
All of this is accomplished in such a way that all parties are able to prove, at all times, which transactions have cleared and which instruments are authorized, without having to store their entire transaction history, but instead by merely keeping the last signed receipt.
More featueres: Smart Contracts (scriptable clauses.), Client-side scripting, with full access to the OT Client API.
https://github.com/FellowTraveler/Open-Transactions 
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