This is a no-brainer! Do not use Telnet, FTP or HTTP to authenticate. However, there are also less obvious protocols, such as POP3, IMAP and SMTP (all email related). SMB and NFS (networked file systems) are also well known to be insecure regarding how authentication is done.
If you have to check email using an email client program, at least select the SSL-tunneled versions of the mentioned protocols. Unfortunately, this requires the cooperation of the email server. If you have to talk to an email server that does not support POP3 or IMAP over SSL, then you cannot transmit your username and password in a secure way. SMTP usually does not require any authentication, so it is less of a problem.