FPCleaner
If you are working with SharePoint Designer, you will most likely notice some annoyances here and there related to cache. FPCleaner is a pretty useful little tool that I found that helped me clear quickly temporary files and other cache files that are most of the times the root of those inconveniences. Some of the best features are the ability to clear the web cache files as when those .web files are corrupted, they will prevent you from opening/closing/publishing a web site. Deleting those files usually solved the problems for me. Another nice feature is that ability to change a last open web setting. Designer by default is opening the last edited web but if your web has problems, well Designer will still try to open that web but will likely freeze.
Custom Workflow Activities
Here is a collection of very useful custom activities you can easily use on your projects. My favorite one is the "Start Another Workflow". This activity let you point to another of your workflow which opens the door to more "complex" SharePoint Designer workflows. This very cool activity really turned out valuable on a recent project of ours. Check out those activities.
Paul Galvin’s blog and custom actions
A great resource for SharePoint Designer is Paul Galvin’s blog. I found a very good post of his where he explains how to create a "loop" in SharePoint Designer in order to create a daily task reminder email system. He also worked on a set of custom actions that you might find useful for your projects especially if you are planning to do some string manipulation.