It drives me nuts that the telnet client isn't part of the default setup with Windows 7 (or Vista, but who cares). I know you should tunnel SSH, but for a quick check to see if a port is open on a destination server (ie to see if SMTP is allowed through a firewall), it's a great tool.
To enable it follow these steps:
- Click 'Start' --> 'Control Panel' --> 'Programs and Features'
- Click 'Turn Windows features on or off'
- Tick 'Telnet Client' (not 'server')
- Click 'OK'
You can then start telnet either directly or from within a command prompt.