I think that future versions of Trisquel (or an upcoming respin) should
include the Croscore fonts by default since it doesn't include Liberation.
Croscore are pretty much the same fonts but more up to date.
The Roboto fonts located at http://packages.ubuntu.com/raring/fonts-roboto
would be a good candidate to backport to 6 or as part of an updated ISO as
mentioned above. They are very readable especially for icon text on the
desktop and window text.
I'd also like to suggest Lato from
http://packages.ubuntu.com/raring/fonts-lato and Open Sans and Raleway from
https://launchpad.net/~elementary-os/+archive/stable
I know that many of these can be installed manually, but having in .deb form
can't hurt as well!
