This is the kind of solution I like: one that solves a simple problem, in a very elegant way.
Problem: Comments in blog posts lack in personality when compared, for example, to comments in online forum software such as phpBB. That is mostly due to the lack of avatars (small images that represent users). Putting avatars in comments would imply registration systems (which would not be universal) and the kind of bureaucracy that is against the blog commening spirit.
Solution: Gravatar. Users register their 80×80 images on the site, and associate it with their e-mails. Blog owners just add to comment templates an tag pointing to Gravatar. This tag contains an MD5 hash of user’s e-mails (which they usually enter anyway).
The MD5 makes it very hard/unfeasible (but not impossible) for scraping software to capture e-mails (don’t know how/if they deal with clashes, but those are rare in general text, and with e-mails there is a good chance that very few, if any, will ever happen).