Yahoo, Google and MySpace are supporting the spanking new OpenID OAuth Extension protocol. I have this on my plate at this exact moment, I am not sure how long it will take them to support this, but for now, I found Yahoo's Contacts API to be the easiest, and Facebook's Registration/SSO to be the hardest.
This is what my todo list looks like:
1) Visit all these websites: http://knowem.com/
2) See which ones have authentication API
3) Implement them.
4) ???