The easiest way to annotate your video is to use the inbuilt tools on YouTube.
There are a few apps which have added functionality like being able to freehand draw onto the video but some of them may cost you a couple of Euros.
Ink 2 Go – is very good and very easy to use. You can have a 15day free trial but the license can be bought for a one off fee of €20.
For iPad, Telestrator app is nice, you can buy it from the iPad app store.
For still images Snagit is a good, free annotation tool. Annotate your picture and then insert it into your video.
For Windows users, Demohelper lets you annotate parts of your screen and may be useful for technical tutorials.