Is there a After Effects like software for Ubuntu (or for free)
Adobe After Effects is a compositing, compositional and post-production video tool. If you think about green-screening, those cool titles/credits, overlays with people's names, infographics... it's all that stuff, and a lot more. Like motion photoshop, in full hd.
There is lots of high end (and sometimes quite expensive) video software for linux.
There is a wikipedia page that has a list of available software (both commercial & open source) for multiple operating systems.
One interesting software project which is not on that list yet is Ramen that is designed for 64 bits Linux operating systems.
Although this is a rather old question, for the benefit of new users / searchers, I think blender needs a more prominent place in the answers than as in a comment. With the finalization of project mango, called Tears of Steel, blenders abilities in regard to post production are stronger than ever -- the node based compositor is very powerful indeed. You need to get used to blender, but it's worth it.
Update: Blender 2.69 was just released a couple of days ago, with many improvements in camera tracking, motion tracking, ...
I've been an After Effects user for 12+ years and I have not found anything really comparable on linux platforms. Cinelerra is probably the closest and looks promising, but I have little experience with it (basically since my clients are dedicated to AE). The other apps listed by others above are primarily video editors; and as I'm sure you're aware, that is not After Effects primary niche.
Cinepaint, developed for the motion picture industry says that they may eventually add features in the direction of AE.
There's PiTiVi which is included by default. And there is LightWorks which is opensource.
