Gravatar (shared avatar)
I am in the process of working Gravatar into my WildObs site. Gravatar provides “hosted user icons” (avatars) for comments on blog/forums/social networks. I applaud that (1) ‘cos I don’t want to host/manage/backup (2) I wanted to save myself writing the image crop (3) folks can use what they’ve already created.
They have good integration support, which was key, and I used the simple Ruby On Rails plugin. I love their “rating scheme” (PG, etc.) I’d not have thought of that. Perhaps obvious in hindsight.
I’d thought of writing this service, if it did not exist. It seems a great scam. Sure, their privacy policy states they don’t “sell your e-mail address” but what about capturing (perhaps selling?) your tracking information to folks who already know your e-mail address? They become a central point of collection for the many blogs you comment on, and other sites where Gravatar is used.
What is impressive about their integration is they capture all e-mail addresses on sites that implement this using their tools alone, and not just the folks who use their service. Again, great scam, and I cannot fault them for their execution there.
So why am I using it on WildObs? First, same reason as I use OpenID (a similar central tracking service, IMHO) that folks are willing to give up privacy for convenience. [My site is one I love, but is likely to be low on many folks priorities so I need to make it convenient.] Second, I’m not my users privacy cop & I make them opt in not opt out. Third, I already use Google Adsense, so doubt it matters much. Forth, I like to believe I support the idea that “the more marketeers know about you the less crap the ads will be”. 
Ok, I am coding it so by default one gets a local WildObs avatar, and one has to request the use of a Gravatar. That way I don’t blindly leak all e-mail addresses.
BTW: OpenID is already (optionally) passing profile information with a new signup, I have to assume “link to avatar” is going to be in there next. It just makes sense.