The second most useful blog I read is the Desiring God Blog.
Several people, John Piper included, write for this blog. They write from a very biblical perspective. Most posts are biblical and theological but also practical. They write on Christian leadership, Christian living, and church leadership topics. They give the reasons behind their beliefs and decisions.
I think that I read and save Desiring God posts more than many others because they give reasons and help me think about issues and decisions. I love blogs that make me think.