At first, if you want to implement the server side of jQuery Ajax calls, you can do this with either ASMX or WCF services. You can find a short comparison between these two here. WCF is more modern technology and will be my preferred choice for new projects. It can provide you with the following:
- Help you program against an interface
- It will serialize/deserialize objects to JSON for you. No need for JSON libraries
- Provides client methods that you can use (via the ScriptManager). It is also easy to use jQuery if you prefer
As an disadvantage I would say that it will take you some time to learn the technology. I found that proper configuration of web.config was a little tricky.
I usually have a single svc service that serves all Ajax requests. You can implement as many methods as you want in a single service. The services are called from different pages.