I guess that those of you who doesn't solve problems only in his mind use pen and paper to make some notes, eg draw graphs, points, lines, permutations, shortly, everything. It's very convenient, but there are tools on the internet that make this process more comfortable in several cases: for example, a graph editor or a geometry widget (there was a nice page by zxqfl, but it seems to be expired or something). All these services allow to obtain visualizations from some (debug-)print-friendly format, which can save your time.
Once I decided to write a tool for drawing grids (as I remember, it was approximately when I was solving the task from the last russiancodecup). You can find it here. Some usecases are also provided. The default text in the textfield is a brief manual, but you can type something and see what happens.
Below you can see an example of usage (based on the problem from rcc above):
Maybe (not very likely) some new functionality will be added. You can copy and modify any code you'll find.