In iTunes, there are two ways to categorize your videos: as either Movies or TV Shows. Movies are displayed individually, but TV shows are organized in groups, as they would be if they were episodes in the same show. One anonymous Mac OS X Hints reader takes advantage of this to organize all kinds of videos, regardless of their actual source. He uses his Movies library for very large files and puts everything else in TV Shows. So, for example, in his TV Shows library, he has groups called YouTube Videos, Home Videos, Work Stuff, and so on.
To create a group, first make sure both TV Shows and Movies are visible in the iTunes source list. If they're not, go to iTunes' General preferences and check Movies and TV Shows in the Show section.
Next, import your new video into iTunes by dragging it to the source list where these libraries display. In most cases, iTunes will categorize it as a movie. In the Movies library, select the video and press Command-I to display its Info window. On the Options tab choose TV Show from the Media Kind drop-down.
Now move over to the Sorting tab. Correct the Name tag that iTunes applied, if necessary. Next, enter a group name in the Show field. This is the trick: Video files that have the same Show name will be organized into the same group; so, for example, our anonymous Hints reader created "shows" called YouTube, Home Videos, and so on. Once you've entered the Show name, click on OK, and your changes will be applied.
You can then repeat these steps for all of the videos you want to organize. Of course, you can also use this technique to group videos you've already imported into iTunes, assuming they're currently in that Movies library.
Once you're done, look at your TV Shows library in Icon or Cover Flow view. You'll see that all of the videos in your groups are organized behind their own group icons. In Album List view, all of the videos will display, but they'll still be sorted by group. You'll also see them organized this way from an Apple TV when it accesses your iTunes library.