Just a number of online resources about building workflow in SharePoint:
- SharePoint document workflow with Visual Studio workshop
- Part 1: Creating a basic SharePoint document workflow with Visual Studio + download
- Part 2: Introduction to custom ASP.NET forms (association forms) + download
- Part 3: Introduction to custom ASP.NET forms continued (instantiation/initialization forms) + download
- Part 4: Workflow tasks (taskedit form) + download
- SharePoint workflow: error updating a list item
- How to video: Building a multilevel approval workflow with SharePoint (MOSS 2007) and Visual Studio
- Workflow Gotcha - can't use surveys - apparently workflow is not supported on surveys.
- Building workflow templates for SharePoint v3 using Visual Studio 2008 - quick update on this posting - Joel Oleson wrote on his blog that building .NET 3.5 workflows is also supported for MOSS 2007 and WSS 3.0.
- Workflow insights - correlation
- SharePoint workflow series from Serge Luca (currently 10 out of 20 tutorials available
- Creating and Testing the project
- Extending the workflow: checking the amount and setting the status
- Extending the workflow: finding the manager and creating a custom activity
- Creating the tasks
- Creating and Using InfoPath Tasks Forms
- Using Several Task Forms
- Using association and initialization forms
- Bringing data from the workflow to the task form
- Using ASPX forms in workflows
- Reuse, Modify and Debug a Sharepoint Designer workflow in Visual Studio
- Workflow insights - dependency
Shameless plug: If you need some offline resources - don't forget to take a look at Real World SharePoint 2007 which contains 2 chapters about workflow development...
Update: David Mann just pointed to his blog about SharePoint workflow development - I just skimmed through it ... lots of must reads.