Henrik Nilsen Omma asked the community on ubuntu-devel-announce to test Final Candidates of Ubuntu, Kubuntu and Edubuntu images.

First, download your favorite flavor Ubuntu (or choose some less-popular option):

After you download a chosen image, check the md5 sum (it ensures that your copy is exactly the same as the original). Simply type md5sum filename in the terminal. Windows and Mac instructions are here.

Then, read the testing instructions and report your results on the iso testing site (I haven't seen it before... very clean interface :) ).

Testing is coordinated in #ubuntu-iso on freenode (irc.freenode.net).