I recommend Electrum: http://electrum.org/

• No need to download the block chain. Just launch it when you want to make a transfer and it's instantly on. Even better if you enable Bitcoin URI's.
• Deterministic wallet, so just secure the seed in a safe place and you will always be able to recover the wallet even if you lose all backups.
• There are many Electrum servers to connect to, and you can switch between servers instantly. No down time.
• Even when you are offline, you will have access to your addresses.
• Portable.
• Many different GUI options. Even has a text GUI.
• Has many command-line features. Also, supports plug-ins, for instance, to help with storing labels in the cloud.

ETA: Ah, and I almost forgot... You can use the same Electrum wallet from multiple different locations simultaneously.