What Users Do, is a market research portal, who specialise in getting their 'testers' to test out new websites in order for the companies to try and find any issues or website flaws before they launch.
The testing can be done on a laptop or a mobile phone, although certain studies may well want the test to be done on a certain type of device.
The pay varies, but normally it's around £5 per test, and each test lasts around 15 to 20 minutes or so.
There aren';t huge numbers of tests at any given time, but they will email you when one becomes available, and then it's first come first served.
All in all, you won't get rich, but its a few extra pounds in your pocket, but payment isn't instant, you normally get then paid the month after you have taken the assignment.