I used to spend a few hours every week looking for new samples to send for, and would usually end up getting a lot of great ones. Nowhere near all of the ones I sent for came, but I still got enough to keep me happy. Somewhere along the way, things got way too crazy, and I quit looking for them quite as much. I do still order samples, but now mostly just stick to a few sites that I know will send them out, or occasionally manage to get one or two from facebook or something.
At first glance, it might not seem like looking for samples is worth the time, since they are usually pretty small. But a lot of them come with coupons, too, and then they become more worth it. Also, sometimes I'm pleasantly surprised and find a pretty big sample, or even full sized products (we've received full size deodorants, air freshener, and several nice razors over the years). We've also found several products that we probably wouldn't have tried without samples, so it's working for them as well. In fact, my son's newest favorite cereal was found through a sample box that came in the mail in April.
Retailer's websites are probably my favorite place to look for samples. Walmart used to frequently have several good ones, but these days they go fast. I still occasionally get one, by checking the site 2-3 times a week here. There aren't any currently, but check back frequently.
Target also has samples sometimes, but not currently. Their website is here.
I just discovered during the past week that KMart has samples available on their website now, too, though. They have a bunch right now, including Purex, Shout, and Miracle Whip, among others. Their site is here.