I am no longer developing this plugin as I have sold my 6500c. If you want to continue the work set up a web page for it, and I’ll link to the page from this post.
This plugin is only tested through trial-and-error so use it at your own risk. Only Bluetooth is supported.
Previous comments regarding [...]

After browsing around on Nokia’s website searching for iSync plugins I actually found a couple of resources for Mac users.
On the Nokia iSync page you can find information on syncing with your Mac and download iSync plugins for your phone model. Sad to find out, there are no official Nokia plugins for the 6500 Classic [...]

Plugin page has moved, visit: new Nokia 6500 Classic iSync plugin page.
This plugin is only tested through trial-and-error so use it at your own risk. Only Bluetooth is supported. Please write a comment if you find any bugs or want to request any features.
READ THIS BEFORE DOWNLOADING:
Always double check what iSync wants to change/add/delete! As [...]