Buttons on web sites that say "Login" or even "Log-In" (and then, when you're leaving, they say "Logout").
There may be a noun "login" as in, "Jeez, I forgot my login for Yahoo." Whether spell check likes it or not.
I'm pretty sure there is no correct context for a hyphenated version.
And I can't think of a single plausible excuse for nouning "logout". Since, like, they don't ask you for a personal verifier to get out of their system, do they? (Unless... a prison web site? -- "One bad decision may have brought you here, but it is much, much more difficult to leave, bwa-ha-ha-ha!!!")
So, HTML geeks, listen up! Make your buttons two words: "Log In" and "Log Out".
Thank you. Like you'd listen to me anyway...