Armory is an open source wallet management platform for the Bitcoin network. It's designed to work alongside the Bitcoin-QT application, providing additional features such as wallet management and cold storage. It offers additional flexability and features and is suitable for advanced users.
A standalone wallet for Android devices and offers a number of features suitable for that platform including transactions via QR codes and NFC (near-field communications).
This is the original bitcoin client. It is a P2P application geared towards more advanced users, although it is fairly easy to use. The biggest complaint with this client is that the blockchain can take a day or more to download, so if you're in a hurry to try out bitcoin, you may wish to consider another client such as Electrum or MultiBit instead.
Elecrum is a fast and lightweight client and does not require a block chain download. It is designed to be a simple and lightweight client geared towards those new to bitcoin, and is an excellent choice for all but the most advanced bitcoin users.
MultiBit is a secure, lightweight, international Bitcoin wallet for Windows, MacOS and Linux. It's open source and supports a large number of different languages, and is another excellent choice for non-technical users.