You're thinking of Swap Magic, Neomatrix. A friend has it and it's not THAT bad, though you do have to remove the front of the PS2 tray and use a slide card. You need some extra thingie to make it work with the slim PS2.
If you have the big PS2, buy one of those fliptop cases. They are better than using the slide card which can damage the gears in the cd tray. IF you dont havea big PS2, then get the small one, all you need is like two tabs to make it think its closed..
Don't spend the extra $5-9 dollars on the "switch" though. It's a small plastic tab that doesn't fit over the sensor. The tab that goes on the back fits, but it puts stress on the lid.
if its a pstwo the youll need the special switch, however if its a regular ps2 then get the flip top, i got it and it makes things so much easyer with flip top, also good if the tray drive decides to give out.