Are you looking to join one as a tester or use their services?
Did you do a search - if so, which ones did you find and what did you find out about them?
What would make a 'good' site to you?
I can recommend Applause (formerly uTest). They've worked with my company, QualiTest Group, quite a bit. The uTest forums is a great place to start of you're looking to get into testing. If you're looking for a testing solution, I'd recommend QualiTest Group to see what will suit your needs.
Here is a list http://apps.testinsane.com/mindmaps/uploads/html/CrowdTesting-Freel... Having shared this, you may want to evaluate like Phil mentioned in order to know what suits you or what is good to you. Without a specific context, its hard to answer you in a good enough way.
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From my experience:
utest.com - I registered there 4 years ago with some newbie testing skills and haven't gotten any job offer until now.
https://crowdsourcedtesting.com/ - I had troubles with my account and couldn't get it fixed by them.
testcloud.io - worked great, they have a nicely organized platform with issues reviewed prior to client sending; you get payed by bugs found; You can get frustrated sometimes when your bugs are rejected as feature, not in scope, not implemented, cannot reproduce, duplicated and so on. You are required to have a tax id in certain countries and work legaly as a self employed.
https://www.upwork.com/ - previously named odesk; you have to put your cv and skills in there; you see a bunch of jobs, and apply to them together with 100-200 other people; some of these people get selected and interviewed and 1-3 are selected to work for 2-4 dollars/hour. If you get 1-2 years of experience working like this you can raise your hourly desired pay to 10-12 dollars/hour. 10-20%(don't remember exact amount) of the money you gain goes to upwork. Expect to work for free or really low amount for half an year or so.
Also from my experience:
uTest - My very first crowdtesting site. Very good experience with this site. So far the best platform I have worked with. They also have a wonderful community. You need to wait for an invite to a project
Passbrains - When I joined I think they were also just starting out because I felt that the platform and forums were still lacking. I login from time to time to see what's new. Also need to wait for an invite
Bugfinders - No need to wait for an invite. You can join any project but so can others. No idea about the community.
I just listed these sites because I was able to earn from all of them. I have joined other sites but imo, it's very hard to jump from one site to another because you can only work on one project at a time. Also, if you want to build yourself in that community, you need to spend more time there.
You may have noticed, from uTest to Passbrains to Bugfinders, my mention of the community disappears. Maybe that's why I prefer uTest more, because I can talk to other testers. But because I had to wait for invites, I looked to other crowdtesting sites for more projects, just for the additional $.
I have not used them personally, but I have heard they are OK. http://www.testbirds.com/
I keep a list here:
Sorry for having some of the descriptions in Hebrew - but the names & links still good :)
My personal recommendation - #testbirds #crowd #testing TestBirds