- I would seriously think before "logging in as some other user but myself". There are laws and privacy policies that normally forbid such behaviour without user's approval.
- Please use good passwords. Good passwords take 3-5 seconds of your time and give you tons of security layers.
- Avoid these solutions for servers that are connected to the Internet.
- Please make sure your user does not have an encrypted home or encrypted private directories. The user can lose their data if they don't know their old password.
sudo passwd myotheruser
Use "x" as password.
Then login as myotheruser with password "x":
sudo login myotheruser
There is another solution, using pam.d without password:
Namely, you have to:
sudo addgroup nopasswdlogin
sudo adduser myotheruser nopasswdlogin
Then change the file
/etc/pam.d/login and place this somewhere before "@include common-auth" line:
auth sufficient pam_succeed_if.so user ingroup nopasswdlogin
Again, please use good passwords, you don't want to be featured on websites such as http://www.zone-h.org/ :)