I am doing some work on modular WSDL. I was looking for a good WSDL editor that was low cost as in free. Don't know how much hand coding you have tried with WSDL but it can become a bit tricky especially when trying to troubleshoot. It comes in handy to have a tool that shows you where some of your links may be off.
Altova is okay for syntax checking but to see graphical WSDL, you have to purchase a licensed copy. I had been playing around with Eclipse 3.1 and their web tools for a while doing some interoperability testing. I did not realize at the time that they had a built-in XML schema editor as well as a WSDL editor. Turns out it is not really bad for the price. They have just released version 1.0 of the web tools.
Here are a couple of screen shots to illustrate what I mean by graphical layout. In figure 1 you can see the correct linkage being made. This saves a lot of time.
Figure 2 shows the import schema linked into the messages section. This is also nice in that it lets verify that your linkages are correct.
Here is a site with some more good tips on using XML schemas and WSDL. I also found another site that where the authors are using modular WSDL in their design. It's a good reference if you are just getting started.