Why should I use USP?
So why would people want to use it instead of the standard menu? It has a host of features that make it much better than the main menu, including support for quick searching of all your installed applications, plugins, tabs and themes (to name a few). It can be used for quick access to more things than just your applications. It's also fully customizable, so you can tweak it to look pretty much exactly like you want it to.
Install USP in Ubuntu 10.04/9.10
Open the terminal (Applications menu -> Accessories -> Terminal) and run the following commands
sudo add-apt-repository ppa:malacusp/ppa
sudo apt-get update
sudo apt-get install usp2 usp2-extras
After installation you can find the panel plugin by right clicking your panel, ‘Add to Panel’, ‘Ubuntu System Panel’