Fastmail which is increedibly fast and lightweight compared to Gmail. It feels like going from Hotmail to Gmail all over again, if you're old enough to remember what that was like.

At home I stick to Thunderbird, which leaves a decent local copy with ability to connect to CardDAV and CalDAV which I handle via Baikal.

Hopefully Fastmail will live up to their promises about providing CardDAV and CalDAV themselves, as that would leave me one less thing to worry about.