KeepTube is a quirky, somewhat amateurish program. English is fractured throughout, the interface is non-standard, and the startup page of the program is festooned with Google Ads--presumably how the free program supports itself. (The program does not appear to install any adware on your system, however.) You can use the limited built-in browser to navigate to YouTube (no other sites are supported yet, but the home page promises that KeepTube will support other "famerous" sic Web sites soon), find a video, and click the "Download" button. KeepTube then saves it, with speeds depending on your Internet connection--very zippy on my cable modem. It supports multiple concurrent downloads, so you can keep selecting videos and saving them.
Once you have downloaded videos, you can construct playlists, adding FLV files from any folder to the list, then play the list in the built-in FLV viewer. It has a Spartan but functional interface, allowing you to choose the video size, pause, rewind, and adjust the volume.
KeepTube works. It isn't pretty and the feature set is minimal, but it does the job it claims to do and the price is right--if you don't mind being greeted with an ad-encrusted, 1998-vintage colored-text-on-black-background Web page when you log on. There are other programs which will download FLV (such as the VideoDownloader plug-in for Firefox) files, but KeepTube does provide playlist management functions as well as a built-in FLV player--a nice bonus.