Marsedit - Mac only - Shareware - perhaps the best blog client for mac.

Tumblweed - Air application - Free

WinTumblr - Windows only - Free

Posty - Air application - Free - Supports Twitter, Tumblr and more….

Tumblr for Blackberry - coming soon (hopefully)

aTumble - Tumblr for Android phones.

Stumblr - client for Symbian S60 mobile devices

Kumblr for KDE/Linux (via Fosswire)

Tumblette (iTunes link) for iPhone (via Arstechnica)… more on Tumblr Staff

Tumbla for J2ME phones (old phones like the Sony Ericsson K608i)

iView (iTunes link) for iPhone, dedicated to view and reblog photos (via Mobthink)

also… a tumblr widget for Mac Dashboard and if you use Firefox, the Shareaholic plugin lets you share stuff on Twitter, Tumblr, etc… (via Rd67)

The Tumbler (iTunes link) for iPhone and iPod touch (via Twitter)

What else should I add?

(Ps. a digg would not be so bad)