There are lots of unofficial sites and software packages which let you download any video from YouTube, many of which violate the YouTube terms of service. But what if you want to download the videos you've actually uploaded yourself? It turns out there are two official methods, one of which is much more flexible than the other.
Google Operating System notes that there is an officially supported method for downloading your videos within YouTube itself (from the Video Manager, click on the 'Edit' button and choose the 'Download MP4' option. However, it limits you to two downloads an hour and won't work if you've added an audio track.
A better choice is to use Google Takeout, Google's much more flexible tool for exporting your personal data from your Google accounts and services. If you've uploaded videos, you can download them all as a single archive. If you select Choose services, you can elect to just download YouTube videos, and get an estimate of how large the archive file will be. Handy.
Download the Videos You've Uploaded to YouTube [Google Operating System]