Or maybe paypal is the most useful of them all.??
to be integrated in asp.net c#
I belive Paypal is the cheapest (or almost the cheapest). Along with the cheap fees, PayPal were the 1st (or almost the 1st) to come out on the web as online payment system. IMHO thus PayPal is so widespread!
All the other accounts in the answers are almost as simple to use as PayPal.
But let's look at FEES
Once agin it shows up that prices are a huge marketing axe on the web, much more than in real life.
And to answer to your 2nd question, C# .net integration does not matter here, all these payment gateway can be integarted with just some HTML code in your site.
I say that moneybookers is also used a lot and its simple.
Maybe in the future google checkout maybe is also good, but for the moment is very limited.
Amazon Checkout is fairly simple to work with.
If you’re based in the UK or Ireland, Sage Pay is a solid, reliable and recognisable name to go with.