With their API they have released a few ‘official’ extras to integrate Basecamp with other programs. These include:
Blinksale invoicing Harvest: Time tracking Tick: Budget tracking BCTix: Issue tracker Basecamp OS X Widget SproutIt Mailroom Phonified Tracker Telescope Sundial widget Scout for Tablet PC Project Recon Time Post FreshBooks Cashboard
Know of any more that aren’t listed? List them now. Basecamp Extras – [37signals]