Also if someone is working on windows and cannot install linux then he/she can work on these servers.
They also provide user a home directory but never try to get access to root files it wont be allowed or never try to use shell scripts that can get you root privilidges that wont work either.
Using them requires knowledge of linux commands.
List Of shell servers:
- http://www.grex.org/ : provides text based lynx browser, free webspace, pop3, email
- http://www.celebris.net/: provides services like php, mysql and apache 7mb quota on webspace, IRC access.
So if you want to see how unix works without the hassles of installing it go and create an account on one of these.