Bitcoin-Qt is the original software written by Satoshi Nakamoto, the project's founder. If you aren't sure which program to pick, this is a good bet. This application is a peer-to-peer client that builds the backbone of the Bitcoin network. It is suited for enthusiasts, merchants, miners, developers and people who want to help support the project. People who run Bitcoin-Qt are first class network citizens and have the highest levels of security, privacy and stability.
Latest Bitcoin-Qt version http://bitcoin.org/
MultiBit is a secure, lightweight, international Bitcoin wallet for Windows, MacOS and Linux. It makes performing Bitcoin transactions quick and simple and comes in a variety of languages because Bitcoin is without borders
Latest MultiBit version http://multibit.org/
Armory is a Bitcoin client that works on top of Bitcoin-Qt, expanding it with functionality for Bitcoin power users.
Armory is an open-source, python-based, wallet-managment platform for the Bitcoin network. It was designed from the ground up to provide countless innovative features not available in any other Bitcoin program. No other program can claim to even have half of the functionality available here — Armory is light-years ahead of any other Bitcoin client out there.
Latest Bitcoin-Qt version http://bitcoinarmory.com/