Here is a directory of usability testing companies, both for websites and apps. Note that these sites pay via Paypal, unless otherwise noted. I have filed this directory under side income because while the money is good, the amount of work available can vary widely.
Analysia: Pays $10 for each test.
Enroll: Pay varies per task. $1 payout once a month.
TryMyUI: Pays $10 per test.
UserFeel: Pays $10 per test, once a week.
Userlytics: Pays $10 per test, and it uses software that you must install on your system in order to record tests.
User Testing: Pays $10 per website test and $15 per app test.
uTest: This site seems to be more for experienced hardware and software beta testers, but you can still apply. Pay is not listed.
What Users Do: Pays $8 per test, and it is based in the UK.
YouEye: Pays $12 per test.