As you'll see, I still haven't done anything, mainly because I can't decide. Some have recommended Disqus (or however you spell it) but I quite often find that it's "having problems" when I'm trying to leave a comment. When that happens to me I end up clicking away so I don't want to encourage anyone to click away.
The most common one I come across is on Blogger and involves typing in a number followed by an illegible pile of letters. Honestly, are they trying to make it difficult? I came across one a few days ago and the letters were actually standing sideways to the viewer. While I was impressed with the graphics, the lettering was hardly conducive to leaving a comment.
I've just left a comment on a blog and it took me three attempts. Even the final, successful attempt looked like this:-
Bear in mind that this is about twice the size of the actual captcha image on the blog. I think I put in 6243, but seriously Blogger, what the what?