I use this to allow me to write on the screen. It's great for screencasting. I can circle important points or work problems in a very natural way. I bought mine on ebay for $35. You can also get a pretty good USB tablet from Amazon for around $60. Once you have your tablet you'll need to write on the screen:

• SmoothDraw - I've not personally tried this one. But it is the program used by Sal Khan. (Windows, .NET)
• Sankore - This is the program I use. It is fairly awesome! Formerly know as UniBoard, Sankore is a great compliment to a tablet or even a DIY interactive whiteboard. (Windows and OS X, but not 10.10)
• ZoomIt - It allows you to easily zoom in on a portion of the screen or make on screen annotations. (Windows)
• Sketchpad3 - This is just a simple sketchpad, but if you just want a plain background with writing it'll do. It runs in a web browser so even works on a Chromebook.

Microphones

If you're recording using webcam or screen-capture software then you probably want a microphone to plug into your computer. Most laptops have a microphone built in but their position is not ideal. They often pick up a lot of sound from the computer's fan or from typing. You can get microphones for pretty cheap (under $10). If you have a little extra I'd go for a USB microphone, they tend to have a better audio quality. This is not a necessity however, I went for a couple years with a $6 microphone. To the right you can see a video I made several years ago comparing a couple microphone options.

While you don't need it, you might want to spend a little on your microphone. I really like my USB Logitech headset. The audio quality is good and it's always the right distance from your mouth and I get virtually no pops. Lately I've been using my Guitar Hero microphone. It sounds good, but I have to be careful to avoid popping on my p's. This one was basically free. I just have to remember to hook it back up to the Wii before I try to play. You can get one for pretty cheap.