My favourite Firefox addons

1. CookieSwap
2. Firebug
3. Google Toolbar

.tpl file output using theme function

So, you have prepared the output in one of your module function with all complicated business logic client specified in last meeting.  Your "return ($output);" throws everything on screen. You became glad to see your code worked finally.
And if Table layout is required you pushed all html in $output variable and theming of your own module page become hectic. Better to go with tpl output.