Paypal; Probably the largest provider, and still probably the best from the customers perspective. It works most of the time, you get little spam from it, and when all else fails you can actually get a response from a real person. For the sellers though its expensive to use, and you will often find sellers trying to 'add' 4% *if* you want to use Paypal. This is apparently against Paypal rules, as you would expect, but sellers still do it - You do not have to pay a premium just to use Paypal. Personally. I generally only buy from websites that offer Paypal because I know my details are safe, and that's the most important issue for me these days.
Nochex; Once a challenger to Paypal was quite a good system, easy to use, lower fees, but its now little more than a curiosity. If you had any money in Nochex, I can assure you its gone now.
Western Union; Now, WU is one of those providers who makes their money from those who can't use anything else. I'm not saying that's wrong, but I am saying that WU is not something to take seriously unless you have no credit cards or bank account. For cash transactions, WU is the only viable option I'm afraid. Fees are tolerable, and the service, whilst fraught with problems, does work. If you can't get a credit card, then try for one of the pre-paid cards, acceptance is generally very high and fees are low.
Pingit: From Barclays, Good try barclays, but hard to setup and waaay too late to the party I'm afraid. Cheaper than Paypal for sure, but not well accepted and not linked to any websites so the transaction is a manual one. I can see merit in this perhaps for phone payment instead of giving card details over the phone but that's in the ideal future, wherever that is.
Google Wallet: Now here's a good idea, that just isn't good, which is a shame. You'll see the odd online store accepting 'Pay with Google', but in reality, its bull. You login to google, add your credit card, checkout, receive your confirmation from Google, and then nothing happens. You wait a few days, then contact the merchant who didn't receive the order. Not big and definitely not clever. Tried a couple of times with exactly the same response. I'm sure Google will blame the merchant and the merchant will blame Google, but who cares - it didn't work and I won't be using it again thank you.
Amazon: I mention amazon because it is one of the largest online retailers and having an amazon account and surviving the torrent of spam it produces is something common to many of us. You can get most things on amazon, and I've spent a load of lolly this way without a single issue. Storefront is slick, payment is easy, fees are low. Can't ask for more (oh, maybe except for allowing amazon account holders to pay via ebay, but that'll never happen).
In Summary, Paypal good, Amazon good, everything else, Mmmmmmm, not good. Every time you put your credit card into another website, that's another possible leak of your card details that you've got to worry about, and don't think it won't happen because eventually it will. The bank will credit you in the end, but meanwhile you've got no cards, limited cash and everything becomes difficult.