That's pretty much a contradiction in terms. There are tons of such scripts around. The best ones are a bit complex. They have to be in order to do all of the stuff you want (and usually potentially more), and to do it cross browser.
However, most of them are also fairly simple. That is to say that their (basic, at least) syntax is pretty minimal. You just install their script(s) and style(s) in the head of your page and perhaps add a short on page initialization script, then add some attribute(s) to the links that you want to have activate the effect.
To my way of thinking it is a big plus if the code allows you to import new links that use the activating attributes to the page and have those links be immediately recognized. But often this isn't so important.
I have yet to see any script of this type that doesn't have at least some minor issue(s).
As a result, I wouldn't recommend any of them. Rather I would say, if you want this sort of effect, do some research to find some that at least claim to have the features you want. Try out at least 4 or 5 of them. See which one(s) are best for you and your coding style and the overall look and feel of your page(s). It's a bit of work. But if you just take someone else's recommendation, you won't know if you have the one that's right for you.
I actually found a pretty good one this morning. It work great but the only problem is that it only 'freezes' the page behind it. Because it is launched within an iFrame, it will only freeze the frame. Do you know a way to make it apply to the parent page as well?
Here were I got the script: http://gabrito.com/files/subModal/