When I started using Mint late last year after reading one of MMM's articles, one of the first things the program came up with was rewards credit card offers. After some googling, I decided to apply for a couple of them. We have fairly heavy expenses right now as I'm renovating a house, our youngest daughter's headed to college with some attendant travel costs, and we're taking a last vacation with both girls in a couple of weeks.
To keep it short, I've racked up enough points and bonuses to cover two round trip flights, all of our hotel stays for the vacation, and I still have Discover points in reserve. All of this with no change in spending, just buying what I was going to purchase already on the cards.
The topper was when I got curious about my longest running card, a Visa from our local credit union. I went and checked, and they had a (very unadvertised) rewards program and I'd accumulated 50,000 points on it over the years. It just paid for a finish nail gun I need for the house.
The challenge now will be to keep on top of them and cancel a couple of these next year before the annual renewal fee comes due. I'll probably take a look around then for the current bonus points leaders and try to repeat the process. It takes a bit of organization and planning, but it's been worth it so far.