SP Workflow Migration Tool allows to migrate workflows created with Sharepoint Designer from one site collection to another. It also allows to migrate site content types across site collections.
It's developed in C# as a WinForms app.
Please note, that the tool has to run on a Sharepoint server.
I'm working on a second release that can also migrate across farms so that it allows you to develop workflows on a test system and move it to production afterwards.