Here’s a very detailed article from WPMU on various tactics and options.
Even if you’re running a private blog, your file attachments aren’t as secure as you might like. Your or your users can innocently copy-and-paste a link to a PDF file into an e-mail … which gets passed around … then somebody puts it in a Twitter or Facebook post … and before you know it the file you wanted to keep just for your selected audience has been indexed by Google!
Everyone’s going mobile — are you ready?
TablePress, Polldaddy Polls & Ratings, WP Twitter AutoPost and Microblog Poster.
Both of these are very specialized tools, with more than a few quirks. Especially in Annotum, the learning curve is fairly steep and prone to trial-and-error. But they impose sensible workflow solutions and organize information in a way that a standard theme or plug-in does not.