Log in / Log on / Sign in...

Log in / Log out
Log on / Log off
Sign in / Sign out

My intuition tells me that "Log in" and "Log out" sound better than "Log on" and "Log off". First, there's a better phonetic difference between the two actions, and second, "on" and "off" give the impression of physical states, such as the computer's power. "Sign in" and "Sign out" are also good phonetically, but I've heard the first two so many times that it's hard to get my head around them. But then there's also:

"This is Mike signing out"
"Signing off on a budget"

Ahhh, so many grammatical ambiguities!