Which digital cameras have LEAST lag between pressing shutter button and the picture actually being taken?

Often, if the light is low or if anything in the field of view is moving even a little, I press the button to take the picture and as much as two or three seconds can go by before it actually TAKES the picture -- this is driving me crazy! Is there a digital camera I can buy that doesn't have this shutter lag?

What you're describing is piss-poor auto focus.
Focus, focus, hunt, hunt, hunt, focus, click. And if you're lucky, your subject hasn't left the room yet.
Shutter lag is the delay after the camera has already achieved focus - it's the same in all light conditions, and it's usually around 1/10th second.
There's really no way to improve the AF speed of your existing camera. It helps if you have time to set shots up, so you can prefocus and track your subject for a second or two (constantly prefocusing), and then click the shutter at the decisive moment. But there's not much you can do to speed up the performance of your current camera.
Some point & shoots are better than others, but for a real improvement you'll have to upgrade to a digital SLR.
Good budget models include:
* Pentax K110D (bare bones model)
* Pentax K100D (as above but with built-in image stabilization)
* Nikon D40 (bare bones but with a better upgrade path than Pentax)
* Nikon D50 (better autofocus, more lens choices)

The cameras with the best shutter lag are the DSLR's. In extremely low light, any digital camera may run into problems focusing or setting exposure, but DSLR's are better at handling this. As far as anything moving into the field of view while you are shooting, the AI focus modes in DSLR's will also be better at handling and adjusting focus.

if you are talking point and shoots its Casio.

Digital SLRs have a shutter lag of just a few milliseconds.
Go to www.dpreview.com or www.stevesdigicams.com. Their tests measure reaction times of many cameras, including point & shoots.