• 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)
• also… a tumblr widget for Mac Dashboard

(list via hacks)