I've been very impressed with this screencast.
Is there a simple way to create a professional-looking screencast using open software on Ubuntu? I'm mainly interested with good-looking transitions between images and videos.
Kdenlive is easily the best video editor currently available for Ubuntu. There is also a gtk-frontend for MLT framework (OpenShot) but it has less features and worse user interface.
Kdenlive supports wide array of video formats, images and transitions. Rendering tool has ready presets for most situations like Youtube, DVD, different mobile devides, etc. so you don't need to be an expert of video compression.
You can get it from Ubuntus repository but I'd suggest you get the latest version from project site. They even have a repository set up for Ubuntu.