iPhone: Spark is one of our favourite email clients on the iPhone, but it's still in its early stages. Today, they're adding some new features that make it a bit better, including HTML signatures, support for aliases, and a new set of gestures.
Alias support seems like the big one for a lot of people. You can now add aliases for your Gmail, iCloud, and Yahoo accounts. You can also now move email between accounts, which is handy if you have a lot of them. The update adds some new gestures as well, where you can swipe to send emails to Evernote, Dropbox, or a handful of other services.
Spark (Free) [iTunes App Store]