My top five most used apps not already include in a default install of Ubuntu:
1. Thunderbird - mozilla email client
2. Amarock - Music management software (default in Kubuntu)
3. Pan - Usenet reader
4. GoogleEarth
5. Conky - Desktop system monitor
Other than these a few others most everything you'll need is already available for a noob. As you get deeper into linux that of course will change.