Google's Chrome OS has slowly added more and more packaged applications, making Chromebooks much more than the internet appliance they started as. Apps can be added through the Chrome web store, and partial support for Android apps is available through the App Runtime for Chrome (ARC).
Chrome OS lacks the ability to do very specific things that traditional desktop OSs can, such as burning CDs or DVDs, though the frequency with which this is a common task is far less than it was 10 years ago.
Killer feature: Extremely hardened security makes the prospect of viruses on Chrome OS less likely.
Price: Free, though it is only available on Chrome-branded hardware from various OEMs, starting at $149. This makes it a prospect of purchasing new hardware, instead of an in-place software upgrade.