Very fast and lightweight module which generates very short links which allows one-time login. Each link has expire date and after this date all not used links will be deleted. After success login user could be redirected to existing internal page. External links are blocked.
One-time login links looks like http://example.com/g47Gv2K3. They could be sent in SMS, Email or even directly typed in. At the same time they hard to guess and their lifetime is short.