To save you time, I've created the following comparison charts allowing you to cut to the chase and view ratings out of 10 for key features plus compare the free features. Click the appropriate link to read a fuller review.
Note - All free personals sites are FREE to join and get the basic membership.
FREE Features Comparison
There are usually two types of memberships:
Credit based or Recurring (Monthly, 3-monthly or yearly).
With the credit system, it offers a way to commit only how much money you want to, but still be able to communicate with various people (any good dating agency will only charge a credit for the first contact between two people, with subsequent contact between both people being free).
The monthly systems usually have a range from anywhere like 9 to 30 dollars a month - features vary, but most will allow you to contact as many singles and exchange as many emails.