I search my own blog a lot – I'll want to find previous posts to send to a client, link to in a new post, or just refresh my memory about something I've written.
Everything TypePad recently led me to the Lijit search tool (before that, I was using a clunky tool that really wasn't meeting my needs), and so far I am absolutely loving it. It will also search other sources like Twitter.
Plus, there are some really helpful statistics, such as the top search terms that brought visitors to your blog, and the terms people searched for on your blog AND DID NOT FIND (from the folks at Lijit: "Hint…it's a great way of figuring out what you should be writing about!").
Check the right-hand column of my blog to try the tool, or visit the Lijit website to sign up for your own account. [Update December 22, 2012 – I am no longer using this tool.]