Running latest version of KDE

Ubuntu repos can be used, but there might be an occasional non-free package.
There are main, restricted and universe, multiverse repos.

Main and restricted, according to the licensing policy, are the closest thing to fully free software.

Here is a link to their policy about including software in the repos.

Universe and Multiverse repos will contain non-free software, because to be included there packages does not have to meet any condition from the licensing policy at all!

You also might want to use the libre-linux repository from jxself, because Ubuntu contains a kernel version with blobs. In order to run the newest version of programs it is sometimes required to update the kernel, so you should watch out to not install the official Ubuntu kernel, but one from jxself.

Here is a PPA providing KDE 4.10. You want the precise "serie" because Trisquel 6.0 Toutatis is based upon Ubuntu 12.04 Precise Pangolin. As Cyberhawk points out, you are on your own to check whether the software provided through this PPA is free (and notice that proprietary software might be added to it later!). That is the problem with using software repository outside Trisquel's control. The people ruling the repository may not be as concerned with freedoms as the Trisquel project...