Again, it depends on what you want to do. Here is the easiest way to install the popular ones... This should work on maverick but it may be different.
Go to System>Admin>Software Sources
Put in your password.
Click the "Other Software" tab.
Click "Add"
Paste this line without quotes "ppa:tualatrix/ppa"
Click "Add Source" and then close. Let it reload your repositories.
Open a terminal and type "sudo apt-get install ubuntu-tweak" (without quotes)
Password and let it install.
Go to Applications>System Tools>Ubuntu Tweak.
Go to "Source Center" and there are a ton of repositories. It might flash something about untrusted PPAs but it's fine.
Here's the thing with Ubuntu. They're slow to update software. In their own repositories, they wait until everything is ironed out and they make sure things are 100% stable. That's all good but you want the latest stuff. You also have access to software unavailable in Ubuntu.
Enable the sources you want, click the software. Install... done.