YouTube's playlist features are pretty neat. However, having them tied to accounts can be a drag when you just want to share a few videos. With this URL trick, you can string a few videos together, no account required.
As tech blog Digital Inspiration points out, you can create custom playlists by taking the video ID (the collection of lower- and upper-case letters at the end of regular YouTube URLs) and adding them to this URL, separated by commas. Replace each instance of "ID#" with a YouTube URL:
This URL will work much the same way any other playlist would. You can embed it or share it with the link. All the normal playlist player controls will appear. The only difference is that it won't be attached to your account forever, and won't stop working if you delete that account.
How to Create YouTube Playlists without Logging In [Digital Inspiration]