For SSL's Name Cheap free comodo is a great way to go and I use name cheap a lot because of this however the SSLs are limited "you get what you pay for".
Verisign is probably the biggest SSL provider and almost every shopper has seen their logo and has trust based off that.
Comodo is more affordable for the same services however people are not as aware of comodo as they are verisign.
Depending on what you do, amount of orders, how much the orders cost should help you determine what type of ssl you need.
There are many other providers however companies such as GoDaddy, I am sure they resell from a provider such as verisign, they buy the .com domains from verisign which is the ICANN registry for .com
Hope the above info has helped.
Good luck!