I maintain two private password protected blogs.
One is strictly work-related. I use it to it log the various reference questions which I receive. This is an important part of my current position, we are actually audited on it this (sigh, little did I know when I got into librarianship that we too could be audited). My blog of questions and answers is helpful because it is very easy to use, gives me a time/date stamp for each question, allows me to mark some of the resources I point people to via hypertext and track follow up work via comments, and most helpfully, and lets me do full-text searches of my queries. I can also share my work with my supervisor. I know there's expensive software which will do all this and more, but the thing is that this solution is simple and works well for me and I don't need to pay anything extra to set up a blog in this way.
I've just set up a private personal blog to keep in touch with family and friends who are scattered across Australia and the world. I am not a fan of the group email. For one thing, people are always changing their email addresses (I'm notorious, this year Gmail is my main email, last year msn, the year before .Mac and so on). Then I know that some people really don't like receiving group emails. Finally, I find it easier to write a blog entry than a group email. Maybe it's because I don't need to worry about being intrusive. People will only be reading this blog if they choose to enter the URL into their browser or news aggregator. Basically, people will only be reading if they're interested, so I can write with more confidence and not feel so apologetic.
Oh, and I've learned that it's possible to subscribe to password-protected blogs. The method for my TypePad blog is: http://[username]:[password]@[subdomain].typepad.com/[blog_ directory]/index.rdf
There's a bit of crossover between this personal blog and my LiveJournal. I expect to save some time by doing some cross-posting. For a moment I pondered just inviting these family & friends to read my LJ, but decided against it. They would need their own LJ accounts to read the non-public material, and that would be too much to expect for some of my very technologically challenged readers. Publishing a private blog is much simpler. People just need the URL and the username and password and it will work.
I am more and more aware that the words which appear in this explodedlibrary blog have a life of their own. Anything I write here has the potential to be read by potential employers, friends, girlfriends and family members. It's both a blessing and a curse. That's why I have a need for a private blog, because I like this medium but I don't want to give up all aspects of my privacy.