Ubuntu live CD's offer tasting full Linux experience without disturbing existing software setup. And there are couple of official & unoffical derivatives based on ubuntu and we are going to examine each and every one of them.
Please note that what ever the work you do with the Live CD's only persists in that session. If you want to use live CD's with persistence then following this official Ubuntu guide on Live CD persistence.
Again Ubuntu is available in two variants desktop & server editions. Both the variants of ubuntu are free and you will get free software & security upgrades.
Official Ubuntu Website
Official Kubuntu Website
Official Edbuntu Website
Official Xubuntu Website
Official Ubuntu Studio Website