I've been on a bit of a video binge lately posting several videos from YouTube here. This can be done several ways, including using the code supplied by video sites such as YouTube. The problem for someone who wants to keep their HTML valid is that YouTube posts the video snippet using <embed> tags which aren't valid HTML. However, like it does for many things, Drupal provides an easy solution. The video filter module makes it easy to put a [ video:URL] style tag in a post and have the video embedded. And best of all it does so cleanly with valid <object> tags.

It's worth noting there are several other modules like the embeded media field that can make the process work in other ways if one is developing CCK types and wants to include videos. So if you're looking for a solution that does even more be sure to check these out.