The delicious tagging innovation is a good one. You can, for instance, browse all the Giles prompts. Fill in your preferred character instead. Or all the Buffy/Xander prompts or whatever. I can imagine that in larger, more active kink memes it would almost be necessary for managing things.
Okay, that's enough of that topic.
Surprisingly useful small tool: Marked, a little app that does a live preview of (Multi)Markdown formatted text. As you know, Bob, I do all my writing in a plain text editor using Markdown for what little markup fiction needs. This lets me concentrate on the prose and not on useless formatting distractions like fonts and so on. When it comes time to post, I just run the Markdown processor on the text. (And my fic archive software takes the Markdown version directly!) This is as easy as choosing the menu item that does it in BBEdit and in Textmate, two great editors. Or I can just copy the html that Marked generates! Even easier.
There is also a trend in MultiMarkdown-aware no-distractions editors, with apps like Byword, Writer, and Macchiato. Or go even more minimal with the positively eccentric Grandview. That last one is a bit much for me. Byword is decent, by the way.
Hardcore tools: I've totally moved over to git for version control on my writing projects. GitHub for Mac takes a lot of pain out of it and you don't have to use Github as a remote if you don't want to. It's a clever and different approach to a graphical interface to git. If you'd rather not use Github, you can do something like stick your repo folder into Dropbox to get offsite backup. Dropbox is sort of a no-brainer all on its own.