In the past, the only way to make YouTube videos semi-private was to limit them to being viewed by 25 people. Now you can mark your clips as "unlisted" and have a "secret" link instead. Here's how it works:
Now, you can mark your videos as "unlisted." This means only people who have the link to the video will be able to watch it. It won't appear in any of YouTube's public pages, in search results, on your personal channel or on the browse page. It's a private video, except you don't need a YouTube account to watch it and there is no limit to the number of people who can view it. You'll get a link when you upload the video and then it's up to you to decide whom to share it with. Unlisted is the perfect option for that class project, video from last summer's family reunion or your secret Broadway audition tape.
Of course, it sounds like you're still out of luck if your friends decide to share the link, but this is still a neat—and rather logical—privacy option to offer users. [YouTube Blog]
Update: Whoops! Looks like the feature hasn't gone live yet, so don't rush to upload videos you want to keep semi-private already.