Sitting here on a murky Sunday morning in Copenhagen your author has had one of these recurring, but important epiphanies. He just realized, again, that the internet is a wonderful place; you think you know most of the interesting sites and most of the places which are compatible with your own wonkish inclinations. Nothing,as it were, could be further from the truth. Consequently I present you Khan Academy (hat tip: Indian Economy Blog) which seems to be me to be an absolutely invaluable resource of knowledge, sharing of insight, and easy to use learning tools. I can only reiterate Prashant's headline; this is indeed a fabulous resource. Here is the intro from the site;
The Khan Academy is a not-for-profit organization with the mission of providing a high quality education to anyone, anywhere. We have 700+ videos on YouTube covering everything from basic arithmetic and algebra to differential equations, physics, and finance which have been recorded by Salman Khan. He has also developed a free, adaptive math program available here.
To keep abreast of new videos as we add them, subscribe to the Khan Academy channel on YouTube.
The entire video library is shown below. Just click on a category or video title to start learning from the Khan Academy!
Khan Academy is run by Salman Khan who is one of these persons that makes you feel humble (and a bit stupid) and I for one love this kind of entrepreneurship for the greater good. Clearly, I would emphasise the math resources  which are outstanding. I sampled some of the videos yesterday night (sorry, but I am a wonk) and for someone who is bent hell on pursuing post graduate studies the Khan Academy provides a welcome break to reading Sydsæter et al's. "Further -ballbreaking- Mathematics for Economic Analysis". You can always quibble on the level of difficulty and for economists well trained in the fine arts of Walrasian (general) equlibrium theory, representative agent modelling and its many modern dynamic derivatives there is probably not much to be had, but for the rest of us there should be a couple of interesting shows. What is more, the format is very nice and the videos are short and concise. Personally I look forward to watching the playlist on differential equations which are important for continuous time optimization problems, but there are also playlists for simple arithmetics and algrebra so this seems to be very versatile.
Now, I have no idea whether this is a well known place already (I would think it is), but it was new to me and I can only say "wow". I will certainly be coming back for more.
 - This is another very good place to go for loads of exercises.