In response to our own concerns regarding dependency on microsoftware among other things, we have created a new multi-platform compositor called ZS4.
This one runs on Windows, Linux and soon Mac OSX. (Zwei-Stein 3 will not be upgraded)

An "alpha" version of ZS4 is already available from www.zs4.net where you will find downloads, videos created with ZS4, tutorials etc. Please create an account over there if you would like to be kept informed of its progress (You can subscribe to different areas of the website if you sign in).

In exchange for not being able to import Zwei-Stein 3.01 projects, and having to learn a new interface, ZS4 offers you:

• User defined output size
• Much improved keying tools
• Uncompressed output including iyuv and motion-png (preserves transparency)
• No 3.5 minute video limitation on Windows (as of v0.928)
• Much greater flexibility with effects
• Limitless sub-arrangements
• 48khz audio

Plus, we're actually working on it, so most bugs will be fixed almost as soon as you report them in our forum on zs4.net.

A couple other notes: ZS4 is only free for personal use. Commercial use license is comparitively cheap though. Details on zs4.net.
It does not run very well on slow machines at this point. We're recommending at least a 1GHZ processor with 512 mb ram. A faster processor would be nice.