Excellent list by Brenton Mayer and Peter McIntyre: http://mcntyr.com/52-concepts-cognitive-toolkit/

I think the list can also serve as a useful index and/or introduction to a lot of LessWrong concepts.

A note of caution: I find that brief lists like this can actually be counterproductive, since they make you feel like you understand the issues when all you did was read a short definition and peg a name on the concept. I'd recommend doing the following: Look through the list carefully and slowly. If there's a concept there that you've read a lot about then you can go on to the next one, although do take a look at where the link points to in case it's to an interesting article you haven't seen before. If you haven't read a lot about the concept then ideally you should click on the link and read all about it. If you're more pressed for time, then at least take a few moments to reflect on each concept and think how it might apply to you. If you have even a slight suspicion that there might be something in the concept that wasn't completely obvious to you before, then click on the link even though you're pressed for time. If you're so time constrained that you can't even do this, then consider just bookmarking the list and getting back to it later. Personally I think it's better to read it later carefully than to read it now and think you understand it when you really don't.