AppExchange for Dummies, for Free.
Go get your free copy of Salesforce AppExchange for Dummies. Even if you're not interested in Salesforce, the Salesforce web services API is one of the best uses of SOAP out there. One of the cool things about it is that it creates a custom object model for your organization. So if you define your own records and fields, it will expose objects with fields that correspond to the fields that you define.
The salesforce.com SOAP API has been available for years, but now they are finally allowing third parties to plug in and offer integrated solutions. There are already several telephony and email solutions that integrate much beyond anything I've seen on other CRM implementations. We use salesforce.com for our own CRM, and we have been very satisfied it. It's interesting that we're such a Microsoft-centric company, yet one of our core business processes runs on (hosted) Java and Oracle. [Live @ Sax.net]