Here is a nice list from 100 Blog Topics I hope you write. The nice thing about this list is that you and I can take the same topic and may end up writing very different things. I wonder whether there is a way to track how many of these ideas get blogged about.
In fact, it may be a good idea to have a blog for each idea with a link to all the blogs about the idea.
Another idea is to add to this list of Ideas for Blogging.