Armory is a cross-platform fully validating Bitcoin wallet, based on Bitcoin Core. Armory features an HD wallet and a wide variety of security and backup features. The project was first announced in January of 2012.
Armory has been going through a transition in development team, making the future of the project unclear.
MultiBit is an open source light wallet that was first published in September of 2011. MultiBit is MIT licensed and built on top of the bitcoinj library, versions are available for Linux, Windows and OSX.
MultiBit uses the lower-security SPV validation mode which gives a user lower security for receiving payments and lower privacy but removes the lengthy syncing time that a full node requires. MultiBit also includes support for multiple wallets and fiat value hinting.
MultiBit has two versions, an older and obsoleted version that was renamed Classic, and a newer version named HD that supports the modern BIP 32 HD wallet configuration. Support from the original developer team has also been dropped for the MultiBit HD version, however a new team run by the hardware wallet manufacturer KeepKey has stated their intention to take over the project.