http://www.markal.com/prod/132/thermome ... stiks.aspx
I was getting ok results with the reflow oven controller. I put a chunk of the 400*F stick on my boards and found they didn't quite melt. Meaning my peak temp wasn't 400*F (ie, well below what it should have been for the Kester paste I was using). The stick itself is like chalk (I used a knife to chip off small pieces to put on the circuit board). When it melts it balls up and changes color. Very easy to tell if you hit that temperature or not.
I bumped the peak temperature up (almost 30*F to 450* ), and my 425*F stick still isn't quite melting. However, the 400*F is, and I'm getting much better results. I wasn't able to find somewhere that sold the 413*F stick, so I'm kinda aiming towards getting the 425 to melt (which is still in my profile).
I think the 413*F and 425*F sticks would likely cover 99% of the users here. The 450*F stick would be good for lead free soldering.
Basically, it's a way to verify that you aren't completely cooking your components, but at the same time, that you got everything hot enough to do what it's supposed to.