Version 3 of the official TubeMate app
Using your app is as simple as accessing the video of your choice through the in-app browser. From there a quick tap on the arrow located at the bottom of your screen. Next step: select factors like resolution and video quality. Your options are: 1920x1080, 1280x720, 640x360, 176x144, etc. Plus you designate which file format you want for your videos: MP4, MP3, WEBM, OGG, etc.
Downloaded videos are stored by default onto your smartphone, but you can also download them directly onto your SD card. Definitively TubeMate 3 is a downloader tool that's really easy to use and has the added appeal of allowing you to download any video file off of YouTube within seconds. This third version of the app also has a very elegant interface that's a major improvement on earlier versions.