I recently visited a customer who asked what their workflow options were for their SharePoint solutions. Some of the workflow options are the following:
1. Out of the box, there is basic approval. You can modify a Document Library settings so that content must be approved before it's posted. Depending on your business situation, this functionality may suffice.
2. Some custom code. In the most recent release of Windows SharePoint Services, we invested a lot of time making it extensible. What this means is that you can create document event handlers for different actions in a document library. Depending on your business requirements, this might be sufficient. For more information, refer to the documentation and http://msdn.microsoft.com/sharepoint. There's also sample code at http://www.gotdotnet.com that you can use as a starting point.
3. Workflow partners - there are several partners who provide great workflow technologies for SharePoint Products and Technologies. Some options you should explore are Captaris's Teamplate, Sourcecode's K2, Nintex and Skelta. There are many other workflow partners as well depending on your needs.
4. BizTalk Server - If you already have BizTalk in your organization and depending on your business needs, you can take advantage of BTS and the BTS Adapter for SharePoint (free of charge) available also at http://www.gotdotnet.com
Net/net: Make sure you understand your business problems before you design and implement a solution. All the options above are -correct- depending on the business situation.