My goal this year has been to save an extra $500 a month, a quarter of my family's income for the whole year, and I've found a number of ways that are working for me.
1. Babysit. I've found a few families on Craigslist and watch their babies in the evenings for date night a few times a month. I used to be a nanny one day a week for a family that let me bring my kids. It was a great deal because our kids got along and I could make money while they played.
2. Mystery shopping. This doesn't pay immediately, but has been really positive for me. I get a lot of things for free that I would otherwise pay for and get reimbursed and paid besides. Recently, I've gotten high end salon beauty products, paint, house plants, an eye exam (I needed it and Medi-Cal doesn't cover it), an oil change, kid clothes, movies with popcorn and soda, a bowling date, restaurant meals, and other stuff by mystery shopping. Don't pay to get started, that's a scam, but there are a lot of legitimate sites. I now get e-mail job offers and don't go looking for jobs anymore, but Mysteryshop.org and Volition.com are a good place to start. I don't have any kind of certification and still get good job offers.
3. Focus groups. These pay pretty well if you're near a city that does them. I find them on Craigslist. Usually they pay $100 or so for an hour or two of time. I've done three this year.
4. Sell your stuff. Ebay, Craigslist, or a good old fashioned garage sale. I've also had luck doing the "drugstore game" getting free stuff by combining coupons and sales, and reselling the freebies on Ebay. Rebates are good in this regard, too.
5. Online survey sites. These take some time, but I've done okay with them. I don't do anything that will make me less than $6/hr, but I've been able to make that pretty consistently, at least an hour a day. It adds up if you're consistent.
6. If you have a blog, monetize! I just started to monetize my blog and I'm earning about $50-60 a month right now. It can definitely grow but I've not been really on top of that yet.