I would suggest you Redhat or Mandrake but thats my opinion of course. As far as links: To download fresh ISO's the best site to do this is LinuxISO.org and its totally free. Be sure to use download accelerators though since the files are big
Knoppix as far as I know is a great distro but its not for a newbie which I think you are in the Linux world.
There are demos and live-evaluations as they are called and mensioned before but they are slow and they come automatically with everything which I would think you wont have when you actually install it yourself.
It depends on the system you are using too, which I think you are using an average normal one most of them would work great.
If you want to try Mandrake you could wait a bit since Mandrake 9.2 is being release very soon. I really like Redhat, although its install is a bit more confusing than Mandrake I think its more complete and better made, but of course its my opinion.
I like both, some have things that the other dont have and vice versa. I think its a matter of the users opinion
HTH
dan