Amazon Mturk is my #1 for making extra cash. I started in February of this year and have made about $2300 so far. I don't really do it consistently and sometimes lose motivation, but it really is a solid way to earn some extra dollars each month. I think it's currently only available in the US and you have to be 18+.
I'm curious what kind of time you're putting in to get kind of payout, and if you have suggestions for how to get the most out of mturk. I tried it for a while last year and found most of the tasks very time consuming for the remuneration offered, not to mention kind of miserable tasks! Maybe I'm just not cut out for it, but I put in a solid effort for a couple of weeks and I think my total payout was something less than $10.
The time I put in ebbs and flows. Some weeks I'll just hang out after work on the couch doing it, or bunker down inside on weekends and do it. I do work a f/t 10-6 job though so I am certainly not at it all day. I guess I could range from anywhere to 1 minute a day to 4-5 hours a day, depending. Some days I don't log on at all.
The biggest tip I have is to use scripts. It's impossible to find anything good just by searching on mturk.com. Turkmaster, turkopticon (for reviews of requestors--I only work for reliable ones), and HitScraper help me get the good stuff. Forums like Mturkgrind and HitsWorthTurkingFor are also great.
I also don't focus on the payout of an individual task. It's how much time it takes. The base acceptable rate for me and more turkers is 10 cents/minute. Yes, this is below min. wage but I can do with work in the comfort of my home whenever I feel like it, and find it pretty easy.
Your numbers matter too. The more hits you have approved, the more you qualify for. I do penny hits to get my numbers up. 1k, 5k, and 10k approved are the general benchmarks. Sounds like a lot but if I can bang out 200 penny hits in about 10 minutes, it adds up quickly.
Also, I like qualifying for the academic studies that are done over a period of days or weeks. I recently did one that was 5 days long and the initial qualifier that took about 2 mins paid .50. Then each day a follow up question took about 30 secs and paid .5. If you competed them all, a bonus of $5. I probably spent less than 10 mins on the whole study over the week so it was a nice little payout.
Last thing for now: turkopticon will help you weed out the requesters that pay very little for your time. If a survey pays 25 cents and takes 20 minutes, that requestor will be bombarded with negative reviews.