I’ve been using the following command prompt for years

server ~/dir :) whoami
victor
server ~/dir :) badcommand
-bash: badcommand: command not found
server ~/dir :( echo sorry
sorry
server ~/dir :) 

It’s a smiley emoticon, but only when the previous command returned without error. If the exit code (stored in $?) returns any kind of error flag then the prompt is reversed.

It’s silly, but surprisingly useful.

Here’s how it works, in your .bashrc simply include the following

## set smiley cursor
smiley () { if [ $? == 0 ]; then echo ':)'; else echo ':('; fi; }
export PS1="[33[01;31m]h[33[01;34m] W $(smiley)[33[00m] "

The interesting point is that you can use bash functions within your PS1 environment variable. This opens up all kinds of interesting possibilities; you could display the total number of running process, cpu load, time, exit code, or even integrate with version control (like git or svn) to remind you of unsaved files.