I've had a couple of folks (Hi, Rich!) ask me about commenting on this blog, so I figured I'd take a moment to explain some of the methods of doing so on a typical Blogger blog.
Here's the comment screen. Unless they're a fan of spam, the blogger will at least have enabled the option in their blog settings, which gives you a bunch of options for identifying yourself in a comment. Note that if the blog is restricted to JUST Google accounts, most of these won't work, and they can be potentially losing out on comments.
There's a bunch of options, and I'll take them in order. However, if you're on a Yahoo! group/mailing list, read to the end :D
First up is 'Google'. If you have a Google account (which means GMail, Google+ or Blogger) that's currently logged in on your browser somewhere, then you'll almost certainly find this is pre-selected and tagged with the name Google knows you by. End of problem: just comment away.
The next two are similar. If you have a WordPress or Typepad account, i.e. your blog is actually hosted on wordpress.com or you have a Typepad account, you can use this option. (Note that just 'having a blog managed using WordPress' isn't enough. You actually have to have it hosted on wordpress.com.)
Next up, AIM - if you have an AOL Instant Messenger account, it pops up a very similar little form and prompts you for your AIM login - again, this'll take you to AOL to verify you're OK with this and set a cookie. This is, I suspect, a special case of the last option, which is the one several folks may want...
OpenID. This is a magic catch-all for a number of other sites that provide OpenID identity services. This is specifically useful for folks on a Yahoo! group.
If you're on (for example) the Lardies mailing list, you will have a Yahoo! login: this has associated with it an OpenID URL.
What you need to do to set this up (before you try and comment) is go to http://openid.yahoo.com/, click on "Get Started" and log in with that account. It will then take you to a page that looks a bit like the one on the left here: it's been a while since I did this for my account, and I can't remember whether it gives you an option to specify things here, but essentially what it'll give you is a URL of the form https://me.yahoo.com/YourYahooLoginName
I've commented on this article via each of these (except the WordPress and Typepad ones, as I don't think I have either of those accounts) just to prove they work.