More Information: I got involved with Arduino mostly control and interface with motors and sensors. I help coach a Science Olympiad team and their robotic events so I have ended up working on a lot of robotics related projects for them. The last two projects for them were a sumo bot and a robot arm. I even built my own H-bridge before I realized that people sell pre-packaged ones. My ideas for future projects currently include pointing a gimbaled camera or trying to build a phased array microphone. Yes, I know that second one is not well suited for the Arduino.
What I have already: A Dueliminova, a good stock of parts (notably always missing the part I really need), and a decent set of equipment minus a function generator (I doubt $50 is going to get me much there).
What I am already considering:
Headers and Jumper Wire (Who can honestly say they have enough?)
SD shield in some form
Motor Driver or Relay board / Shield
Mega (Many of my projects seem to be pin limited, how is the mega comp. to what I have?)
Solid Core Wire (I'm running low)
Small Solar Panel (sick of replacing batteries for our thermometer station)
Perf. Board (Thanks JoeN, you reminded me I was getting low. I was never one for veroboard, but perf board is another story)
Note: As Udo pointed out, because I will be buying books from Amazon anyways this is really equivalent to cash, and thus I could get something anywhere. So I guess the new question is how should I spend $50. I've enjoyed reading your suggestions so far, and if nothing else tell me how you would spend $50.