Because I use these systems daily, I've created my own tag system - a folksonomy - that I use across each of the different tagging applications/platforms I use. For genealogy items I use the following tags:
genealogy (for everything genealogy)
genblog (any blog dealing with genealogy topics)
genapp (genealogy software)
gendex (a genealogy index - like Cindi's List)
genpub (a genealogy publication)
gendata (a genealogy data source)
surnames for each family I'm researching
location names - city, county and state - for areas I'm researching
Every tagging site allows multiple tags. I bookmark every article I write beginning with my site tag (familymatters for this blog) and then topic specific tags. That [and Diigo's tools] is how I'm able to collect my earlier posts for a related articles list. It's so flexible that I can create a list of only my articles or any articles I've bookmarked with a specific topic keyword.
Tags are becoming an important component of your research toolbox so spend a few minutes developing a plan - a folksonomy - of keywords for your research. Anyone who's interested in developing a standard for genealogy tags, please let me know in the comments. My tags are very handy for my research, but "our" tags can help us all!