Ideas on making some good extra cash on the side

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I want to get a discussion going on ideas for making some good extra cash on the side. Anything from producing an extra $100 - 4 grand a month into my pocket and maybe shine light on putting more into your pocket as well. Please don't post websites that do surveys or emails, as those aren't going to net you any real money. I have heard of writing E-Books and marketing them on a site or trying google add words but am not real familiar with all of this or where to begin. Any way feel free to post away with ideas :P