I'd been reluctant to post this in the past as there was already a very good and very flexible version already on here, but combo boxes seem to pop up again and again in the help forum and so I thought I'd just bung this one in here too as maybe it scratches a different itch.
I've used it myself quite a bit and it appears to work, but I can't guarantee there are absolutely no bugs as I have not done extensive testing. I've knocked up an example workbook and added some comments to try and explain how things all work. By default nothing is set and you can either change this in settings or on launching the userform.
Hope this helps someone at lease