I've built a number of OSX app's that integrate with Salesforce.com, all of them are free.

Maildrop - easily copy emails from Mail.app or Entourage to Salesforce
Trapdoor - manage all your saleforce logins on the keychain and quickly login as any of them.
SoqlXplorer - a developer to explore your salesforce.com schema, and try out queries, generate schema reports.
ZKSFoirce - an open source Cocoa library for the Salesforce.com API

Still Beta
SF3 - synchronized contact, events & tasks between salesforce and addressbook / iCal
Quicksilver plugin - quickly search and upload documents to saleforce.com from Quicksilver.
sfdcFuse - using macFuse, mount your salesforce.com document repository as a real OSX volume.

Comments & Suggestions welcome.