There's plenty of good stuff available from PPAs, and they're easy to incorporate into your regular updates. PPAs, or Personal Package Archives, may simply be one-off applications that haven't (yet) made it into the official distribution, or whole suites of software such as KDE 4.6 that come out after the official six-monthly distro release cycle.

(A word of caution: some PPAs may be 'bleeding edge'. If you're leery of this, stick with the official distribution.)

Ubuntu have a great guide to the whole repository schtick, but if you're in a hurry there's really only one command to remember ...

sudo add-apt-repository

Tack the repository name to the end and you're done. For KDE 4.6 for example, we're told to use the Kubuntu Backports which is listed as "ppa:kubuntu-ppa/backports", so ...

sudo add-apt-repository ppa:kubuntu-ppa/backports

... will set up the PPA and fetch its key to verify the software hasn't been tampered with since it was built. Then it's just a matter of ...

sudo apt-get update

... to update your package repository, followed by ...

sudo apt-get upgrade

... in the case of something like KDE 4.6, or simply ...

sudo apt-get install package_name

... in the case of a non-distro package.