While individual blog comments tend to have little or no direct SEO benefit, it's still well worth any blogger or webmaster's while to do as many of them as possible (as long as they are thoughtful and non-spammy, of course). This is because it gets your URL noticed -- not only by the blogger on whose blog you're commenting, but also by many other bloggers reading that blog.
So, you should always be looking for good, popular new blogs. One of the best ways of doing this is to just scroll down the comment threads of high traffic blogs in your niche. You'll find tons of them!
You can apply a similar approach to Twitter. The social networking site is now a very popular search engine in itself. So you just use it in that way. Search for, say, one niche-specific keyword and "blog" or "blogger" and you're sure to find some great new blogs in that niche.
Of course there is still a bit of sifting through results that needs to be done, since not everyone who tweets about blogging is going to be a blogger himself. Still, quite a few of them are.
Just as with Googling, it's useful to mix up the terms as well. If I use keyword combinations like "SEO+blog", "article+blog", "blog+marketing" and then search for various permutations of those searches I get lots of different results. The fact that Twitter is so hugely popular and always frantically being updated alters the mix even more. The results for each search change a lot from day to day, and even from hour to hour in some cases.
The hash tag should be remembered as well. I find a lot of good blogs in my niche by searching for #bloggingtips, for example.
Once you've found these new blogs, you can then look at the lists that they're on. That will turn up still more similar blogs.
Using these techniques, I find that every now and then I turn up bloggers that I've already found via comment hopping. But there are a lot of completely new ones as well. Approaching the blogosphere in a different way means that you'll find new zones of it that contain new clusters of blogs. I suspect that scouring Facebook would be similarly fruitful as well.