Vonage, the international VoIP provider, has just released its Vonage Companion software for OS X. Vonage Companion is available to users who subscribe the the Vonage Pro plan and it basically allows users to use their laptops or desktops to make or receive phone calls. Think of it as Skype but for your Vonage account.
You can use Vonage Companion to record calls, set-up conference calls (or separate conference calls into separate phone calls), selective call block, call transfers and more.
I don't have a Vonage Pro account, so I wasn't able to check out the features firsthand, but flipping through the manual (PDF link) it looks pretty powerful. The biggest benefit to a service like this is the ability to make and receive calls from the road, using your Vonage number and account credentials.
Vonage Companion is free for Vonage Pro customers (existing Vonage users will need to upgrade to a Vonage Pro account) and it works with OS X 10.4.11 Tiger and OS X 10.5 Leopard, though Vonage strongly recommends having the latest security updates and latest OS updates installed. You can download Vonage Companion here (direct .DMG link)