There really aren’t many types of tagging out there but we need applications and websites to use one method of tagging.
Delicious uses tags separated by spaces. WordPress uses tags separated by commas. I believe the tagging system in most Mac applications that use tagging use the comma method.
Application developers should stick with the comma method. Spaces limit the usefulness of tags because in many cases an item needs more than one word to have a descriptive tag. I just tagged this page in my Delicious tags and had to tag it as “writing resources reference”. I’d love to be able to say “writing reference” or “writing resources” and have those be one tag. If I want a more descriptive tag Delicious forces me to use hyphens or underscores (writing-reference) which I think doesn’t look clean.
I hope people stop using the “separate tags with spaces” method. It really isn’t very useful.