This is just a repost of this blog (link) about custom workflow activities that I found really useful.
Here are just some of the things that are accomplished with this:
During my work in Afghanistan for CSTC-A, one of my biggest selling points to them for upgrading to MOSS 2007 was workflows. I really wish there were more workflows packaged but we ended up having to spend a lot of time creating them from scratch. It was time consuming because we were still dealing with post-upgrade things like training for end users. This CodePlex project is really a great tool that every SharePoint Architect/Engineer should keep handy.