Happens to me all the time.

I use Evernote to keep track of the most common bash commands, API calls, syntax specifics, etc. Flash card format, e.g.

Title: Check if object is in a NSMutableArray Content: containsObject:

Are there any applications (mobile, web or whatever) whose sole purpose is to help a programmer capture the program that is in his/her head?

The first thing that comes to mind is UML based applications like Rational Rose. But those seem to have such high barriers of entry and are more geared towards communicating the program to other developers or different stakeholders.