Here is a great free app called Compress. If you keep video on your iPad or iPhone and want to free up some space without deleting your videos then get this app. There are three levels of compression. The two levels offering the higher quality will compress videos at the following levels (these numbers are taken from a 3 min video compressed on my iPad). 262MB to 115mMB using the 720p quality compression and 262MB to 20MB using the 360p quality compression. The quality drop is minimal and I found it difficult to hear or see any difference between the original and compressed version (both 720p and 360p).
Before using the Compress app I had 8GB of free space on my 32GB iPad. I keep a large amount of video on my iPad including skills video and Just Dance video for PE as well as keeping video in student portfolios as part of student assessment in HPE. This means I am always on the edge of filling up my iPad, particularly towards the end of a semester. So after compressing 17 Just Dance videos (took about 15 minutes) on my iPad I suddenly had an extra 2.2GB of free space. This was using the 720p compression option, had I used the 320p option I would have saved myself 4.5GB plus of space.
Note: Video over a certain length (10 minutes from what I can gather) cannot be compressed by the app without the app trimming your video which is not practical.
My main use for this app will be compressing instructional YouTube video and video for student portfolios.
Thanks to @mrKampen who shared a blog post on his iteachPE blog explaining the Compress app.