Playon is a commercial server that aggregates content from web sites using ‘official’ and ‘unofficial’ plugins and scripts. A free version streams your media. Playon channels are, for the most part, aggregators of files posted to internet servers. The Food Network, for instance, is a bunch of episodes or programs that are on the Food Network rather than a continuous stream of programming.
There is a private Playon channel for the Roku. With that plugin installed, you will be able to watch a lot of content that is otherwise not available on the Roku. The free version or Hulu, YouTube, The Food Network, The History Channel, and Lifetime are examples. See the full list of channels here.
Playon is relatively inexpensive. Unlike TIVO, the Playon license does not expire when your hardware fails — you can transfer your license from machine to machine forever. The only limitation is that the license is for the current version of Playon. The current version will not stop working when a new version is releases, but you will not get a free upgrade to the next version. It’s not as limiting as it sounds, you only need to upgrade when Playon adds functionality or channels that you’re willing to pay for and Playon offers discounts to upgraders.
One of the things I like about Playon is that I can use it to watch TV on my Kindle Fire. To do this, simply open the silk browser, disable accelerated browsing (tap the menu button, tap Settings, toggle Accelerate Page Loading to off), then browse to http://m.playon.tv.