Whenever I update (rebuild) a PC with Ubuntu, there are a select PPAs I always like to add.  These give me the extra or more updated versions of software I like to use.  Below, I will list (in alphabetical order) and describe them.

Cairo Dock

sudo add-apt-repository ppa:cairo-dock-team/ppa

https://launchpad.net/~cairo-dock-team/+archive/ppa
This gives me the popular dock type launcher bar at the bottom of my desktop.  It has many applets, is very customizable, and looks great with my OpenGL based desktop.

Deluge

sudo add-apt-repository ppa:deluge-team/ppa

https://launchpad.net/~deluge-team/+archive/ppa
This is my preferred BitTorrent client.  It is a VERY simple, PyGTK based client.

Gnote

sudo add-apt-repository ppa:gnote/ppa

https://launchpad.net/~gnote/+archive/ppa
I always like to remove any mono-based applications, and this is a perfect c++ based replacement for Tomboy.

Medibuntu

sudo wget --output-document=/etc/apt/sources.list.d/medibuntu.list http://www.medibuntu.org/sources.list.d/$(lsb_release -cs).list && sudo apt-get --quiet update && sudo apt-get --yes --quiet --allow-unauthenticated install medibuntu-keyring && sudo apt-get --quiet update

https://help.ubuntu.com/community/Medibuntu
This is "the best" way to add all those nice codecs and multimedia applications to Ubuntu.

PiTiVi

sudo add-apt-repository ppa:gstreamer-developers/ppa

https://launchpad.net/~gstreamer-developers/+archive/ppa
This is a very nice video editor for Ubuntu.  I've used it to put together a bunch of YouTube videos I've uploaded.

TheLupine

sudo add-apt-repository ppa:thelupine/ppa

https://launchpad.net/~thelupine/+archive/ppa
Does this really require a description This is my own personal PPA used for updated versions of pyconnmgr and pyscreenie.

VirtualBox

sudo sh -c "echo 'deb http://download.virtualbox.org/virtualbox/debian karmic non-free' >/etc/apt/sources.list.d/virtualbox.list" && wget -q http://download.virtualbox.org/virtualbox/debian/sun_vbox.asc -O- | sudo apt-key add -

http://www.virtualbox.org/wiki/Linux_Downloads
VirtualBox is a powerful, free and very easy to use virtualization application.  It is based on qemu and can take advantage of VT technology.

Wine

sudo add-apt-repository ppa:ubuntu-wine/ppa

https://launchpad.net/~ubuntu-wine/+archive/ppa
For those much "needed" win only games, Wine is a must have.  It's always nice to have the latest build, and this helps.