I've been going on a bit of a blog commenting blitz lately. It's definitely worth doing from time to time. There are just so many blogs out there about blogging, marketing, etc. They're great because not only can you learn new stuff all the time, they're very conducive to participation. And because they're not super-specific in their focus, it's usually really easy to find something that you can contribute thoughtfully to.
Also, the bloggers themselves appreciate the feedback and often reply. They come back to my blogs and return the favour, too, which is appreciated.
The best thing about comment-rich blogs like these is that you'll find an endless stream of them simply by scrolling down though the comment threads, since many of those commenting are in the same niche.
And here's another good way to find them:
You'll keep seeing the same bloggers' names popping up in the comment threads. So you know that these people been particularly active with this method. So if you do a backlink check on one of their URLs (Yahoo is probably the easiest and quickest) you'll see scores, if not hundreds of the blogs they've commented on showing up in the results. While you'll see some of the blogs you already know about, you're sure to find some new ones pretty much every time.
You just repeat that process, and on and on you go ... You could do this for weeks and never run out of new blogs to learn from and contribute to.