click here to see a list of them. Alternatively, if you are technically minded, you can set up your own Open ID linked to your own web address. For a tutorial on how to do this click here.
Using your Google Account as Open ID
Google Accounts (used for Gmail and other Google services) are now enabled for use as an Open ID. WHen you visit a web site that uses Open ID you will be asked to enter your Open ID "Identity URL". For a Google account this will be http://www.google.com/profiles/googleusername replacing "googleusername" with your actual Google username.
You will then be taken to the Google log in page to enter your google username and password and decide if you want to authorise it to connect either once or permanently.
To test this go to the OpenID test login page: http://www.openid-ldap.org/test.php
Note: If this doesn't work, it may be that you don't have a public Google profile. This is required for OpenID to work. To check your public profile go to http://www.google.com/profiles/googleusername and if there isn't one you can create it here: http://www.google.com/profiles.
Using a Blogger Account as OpenID
If you have a Blogger account with a custom domain (your own domain name) you can use that address (http://www.customdomain.tld) as your identity URL and log into Open ID with your blogger username and password (which will normally be the same as your Google username and password).
Controlling Authorised Sites
You can control which sites you have allowed to use your OpenID by logging into your Google account page here: https://www.google.com/accounts and click on "Authorizing applications & sites"