soapUI
I'm surprised I haven't posted about soapUI yet given how often I use it. It's one of the first web service testing tools I ever used, and I haven't had much need to look elsewhere. soapUI is great for both functional testing and performance testing. They also have a great service mocking feature.
I've never used the pro version, so I can't speak to those features or the support that comes with it. But I will tell you this, the first (and only) time I had an issue with the tool I contacted the developers and they had a build for me the next day with a fix. I was amazed at the turnaround. I'm sure not all fixes can happen that fast, but it still says something about their passion for the tool and the people that use it.
You can checkout the full feature set here: http://www.soapui.org/features.html
December 27th, 2009 - 18:12
I used the Pro version quite a bit and I found it extremely useful when doing data driven testing where the data you use is coming straight from the database.
The support to building your assertions is significantly better in the pro version, allowing increases in your scripting efficiency.
Groovy also give a lot of flexibility and allows you to create custom code for whatever you want.
But, most be people will be satisfied with the the free version, it truly is a gem.