Question: I am looking for a starting point for my project, preferably using popular systems (ones there is a lot of support for). I have an Arduino Uno, a Raspberry Pi, and a lot of willpower :) Anyone here built a project using the systems above?

Observation: I'd like to start with a simple line-following vehicle and build up afterwards.

Answer: 
I built a line following robot with an Arduino before. It was really simple to do and all we used were color sensors on the bottom inputted in the Arduino, and then of course some motors for the wheels. 

But using an Arduino allowed us to have plenty of room for other components we wanted to add on to make our robot do more things. 

Also, if you want to see some line following code we used just ask in a comment, but it obviously depends on your setup with the sensors and how you want it to turn at intersections and things like that.
If you want to go further than using microcontrollers you could run ROS on you [Raspberry Pi](http://www.ros.org/wiki/ROSberryPi/Setting%20up%20ROS%20on%20RaspberryPi).

I have build several robots and a couple of other projects using a [RoboCard](http://robocard.dk/) (site in Danish, but can [be translated](http://translate.google.com/translate?js=y&prev=_t&hl=en&ie=UTF-8&layout=1&eotf=1&u=http://robocard.dk/pages/home.php&sl=da&tl=en)). The RoboCard is build around an ATMega, so that certainly is a viable route.