Which login systems do you prefer?

There's the "popular" choices, for sure, but which login accounts you prefer to use, and prefer to have supported when you're a user of a service?:

Choices are (choose as many as are relevant for you):

a. Facebook

b. Google

c. Twitter

d. GitHub

e. StackOverflow/StackExchange

f. custom username/password for the site in question

g. [fill in your own]