MORPHTOOL is an interactive GUI, with which you can try different morphological operations and structuring elements on your image.
This GUI supports most morphological operations, including those (like BWHITMISS) that can take multiple STRELS. All STRELS, including arbitrary ones, are now supported. You can easily see the effects of changing the size or shape of your STREL, or of trying different morphological operations. You can also "activate" the modified image to string operations.
This has been in the works since mid 2007, but has been tested through R2011a.
CAVEAT: I'm still tweaking some aspects of this GUI, and know about a few issues and holes in functionality that I haven't had time to address yet. Nonetheless, I use it frequently, and everyone I've ever shown it to has asked me for it. Comments/suggestions/collaborations welcome.