Boxcar for Mac now available for beta testing
The Mac version takes a place in your toolbar. When you click it, it shows a drop-down list of all your notifications as they arrive, in real-time. You can set it to play a sound for each new notification, and if you have Growl installed, Boxcar is able to notify you visually each time too.
Boxcar is not available from the Mac App Store. According to Jonathan George, the app's creator, it will be in the future though -- possibly once it's out of beta. George also laid out Boxcar's mission to TechCrunch, and it certainly looks like you can expect Boxcar to pop up in many more places, since he believes in "delivering messages to people -- not devices; [...] to you, wherever you're at. That will be on your phone, tablet, desktop, TV or car".
Download Boxcar Beta for Mac