Hello I have downloaded a few  3d objects from the free stuff section and am a bit concerned about the rules surrounding its use.

1. Am I allowed to use any I want,,, put them in an animation and release it onto youtube?
2. Do I have to mention each person whos materials/objects I use in a project?
3. I found an awesome poseable Alien (Geigers Alien) what is the story with this ...
   am I allowed to make Alien 5 ?! And do I have any rights to do this? 

There must be some readme.txt with those files explaining what can be done with them.
Generally, you are free to use the objects for non commercial purposes ( you don't make money with the objects), unless stated otherwise.
So you can use them and post a movie on youtube, provided you don't get paid for it.
this is the general case, but the readme or eula can state differently, and so you really must read it first.

Cool I will check that... I don't want to start ripping off other peoples work.

 It's best to keep all your free stuff separate unless the eula says it can be used for commercial renders with no credit given.
Love esther