If you have a second- or third-generation Apple TV device, you can use your Mac to push content to an HDTV via AirPlay. Many people with iPads have been doing this for a while, and in Mountain Lion, Apple now lets you push 720p content from your Mac to your TV through Apple TV.
You can now view any Web-based, streaming video service that you have on your PC on your TV as well. As you can see from the screenshots below, it's easy to configure and set up. It's also easy to become addicted. This is one feature I missed when I switched back to my production box, which runs 10.7.4 Lion.