I use avant window manager over cairo dock, just a personal preference. You can get some themes and icon packs for it and make it look pretty awesome. You can get it in Ubuntu with
sudo apt-get install awn
Or just through Synaptic by searching 'awn' or 'avant'
Another thing you might want (which I dont use anymore) is conky. Conky allows you to display information right on your desktop, check it out you might find it useful.
Pretty sure its in the repo's for Ubuntu or just get it from http://conky.sourceforge.net/
Gnome-look.org is good for themes and what not though the way they are packaged doesnt seem very consistent to me and it usually turns into a hassle getting icons etc to work.
The main one if your system is decent would be Compiz. It give all the effects you mostly see on youtube etc Things like window effects, the rotating desktop cube, transitions etc. If you have a decent pc you will barely notice any performance hit and if its setup right you can 'wow' most windows users who havent seen linux before
It is basically installed on default, im not in front of a ubuntu box at the moment but you basically go to "System -> Appearance -> Desktop effects (I think) -> Advanced".
To get advanced options install
sudo apt-get install compiz-settings-manager