Vanillicon is really geared more towards its Vanilla Forums software, but they made it easy enough for anyone to use the service. Here’s how:
http://www.vanillicon.com/<md5 hash of email or username>_<image size: 50|100|200>.png
This is very similar to what Gravatar does:
One major difference is that Gravatar supports image sizes up to 512px, by easily just adding ?s=### to the end of the img string.
The Vanilla Forums team felt like that they didn’t want to use the third-party systems that already exist, or Gravatar, and they wanted to build their own.