There is no standard way to tag videos. In audio, where pretty much everyone has standardized on ID3 tags and other formats like ogg, there are utilities to convert tag information. In the video world, there are too many container formats (mkv, amov, ogm, mp4, mpg, wmv/asf) which all have a different (or none) tag scheme. There are hardly any tools to manage these tags, let alone convert between them. Most video players don't even know how to parse the tags or display them to the user.
That said, many photo/music organziers such as J.River Media Center, Picajet, AcdSee etc include various levels of support for tagging videos. Almost all of them can import video collections and extract thumbnails to display them in the library.