Useful browser plugins for openid authentication?

Answer 1

You could try Sxipper. It provides intelligent automatic form-fill, including auto-login.

From the Sxipper FAQ:

How does Sxipper support OpenID?
Sxipper remembers your OpenIDs and presents an overlay. You choose the one you want to use and login with one click. Sxipper also helps protect you against phishing.

Answer 2

VeriSign (ick)'s SeatBelt plugin: https://pip.verisignlabs.com/seatbelt.do

Ideally, the plugin would allow a higher-level of authentication. I know something like this was planned for the OLPC.