Not sure if anyone here has mentioned this yet, but Plex does even more than give you access to local media on your network. It also has its own channels, similar to Roku channels. One of the channels is a YouTube channel, which means you can actually get access to YouTube on your Roku. Of course, you have to first open the Plex app, and then open the YouTube channel from within it, adding an extra step, but that's it.
Plex also has a Hulu and Netflix channel of its own. The way the Hulu channel works is Plex loads the stream up on your server PC, transcodes it, and then sends it to your Roku (or other device using the Plex app). What this means is you can get access to the free version of Hulu, rather than needing Hulu Plus. It also means if you're in a country that doesn't have access to Hulu, you could find a friend in a country that does and have them set up Plex on their computer with the Hulu channel and have them share their Plex server with you (you can add multiple remote servers to Plex). This means you can now watch Hulu, even without having access to it in your country, because it's the Plex server in the other country that's loading the initial stream, not you.
Anyways, when in the Plex app on your Roku, go all the way down to the bottom row of icons (where the Settings icon is), and go to the arrow on the far right; click this to load the Channels store. Note that certain channels, like the Hulu and Netflix one, require your Windows computer to be running Windows 7 or 8 (no XP), or your Mac to have installed a certain plugin (from the Plex preferences panel on your Mac). They're not available through a Linux server, though. This is because those channels use Flash or Silverlight, and it needs to be able to access the video and audio streams within them. YouTube will always work though because they provide their videos in HTML5 as well as Flash.
ALSO, another way to get access to YouTube videos (and some other ones) through Plex is by adding the bookmarklet script to your browser (or the extension to Chrome). When on a video page in your browser, you'd click on the extension/bookmark and it'll add the video to your myPlex Queue. Make sure you have a myPlex account, and that your Roku is logged in to it through the Plex settings. Your queue will then show up on the Plex main page, and you can watch the video from there.