Technorati now claims to be watching over four million weblogs. That's a lot of blogging going on out there.
I wonder how many of these are real weblogs (active, not duplicates, not just search engine spam). Feedster crawls less than 1M. Blogstreet lists only 140k. blo.gs lists 2.8M.
It seems that no one really knows the answer. There's even a blog dedicated to trying to figure out how many blogs there are out there.
In the end, it probably a lot depends on how you define active blog. One good metric might be the number of unique weblogs that were updated at least once in the last 90 days. The NITLE Weblog Census was the closest thing I could find to this. They list 1.3M active English weblogs (and 2.2M total worldwide). Their methodology sounds pretty solid, so this might be the best estimate of the number of blogs out there.
[Thanks, Micro Persuasion, for pointing out the Technorati stat]