Since I purchased my 3rd generation Apple TV on March of last year(2012), I have been looking for a way to watch flash videos/streams from the internet on it, I have been testing media services that could work through AirPlay, and even tried to create my own, until I settled for XBMC with the Navi-X plugin. I have tried Plex before XBMC, But it's focus on local files seemed like the opposite of what I wanted, so I went on for XBMC.
It all changed last month, when I found PlexConnect. PlexConnect is a Plex Client for Apple TV that thinks different. It replaces the Trailers app with a Plex Client made just for your Apple TV and without jailbreak. You change the DNS on your Apple TV to the IP of the computer running PlexConnect (I run PlexConnect on the same computer that runs my Plex Server), and PlexConnect will reroute any traffic to the Trailers app to the Plex server, and using Python, it converts any response from your Plex Server into an XML your Apple TV can understand.
I have been using PlexConnect with my Plex Server and it works really great, It has all the things I wanted, including streaming media from the web using the unofficial Navi-X Plex channel.
I highly recommend PlexConnect if you are looking for a way to watch streaming media on your 2nd or 3rd generation Apple TV, it's great and it's totally free.