It is safe as long as you don't use it in a hot environment and you aren't doing any terribly heavy processing. I use it every once in a while to play with MacSaber. The current version works on leopard by the way.

Not Leopard compatible.

To the people who are worried about their laptops being damaged while running closed:

All MacBooks are officially able to run with their lid closed while a mouse and monitor are plugged into it. This was a feature included by Apple, not a hack- and in addition to this, the system can run at full capacity while doing so.

This program simply let's you do the same thing without a mouse and monitor plugged in.

when i close my lid while running this, upon reopening the lid, my macbook crashes.

Wonderful. It's the only one I've found that works all the time.

As many people will warn you: make sure your laptop is meant to run with the lid closed. Otherwise, closing it may cut off some airflow, and overheat it. I believe all Powerbooks are good for it though.
I'd also recommend keeping processor performance at reduced, to cut down on the heat (just to be safe).

First, check if your model laptop has the option to run with the lid shut while plugged into an external monitor. If so, this should be safe to do.

Can anyone investigate whether this is safe cause this would be an awesome app if it weren't dangerous. I feel like Apple would have included this as an option if it were safe to do.

But has potential to destroy your screen if your computer is running hot.