Nikon's MC-20 versus MC-30 (and MC-36)
A lot of people have some degree of confusion when it comes time to get a remote cable for their Nikon camera. For modern cameras, Nikon makes the both the MC-20 and the MC-30 cable. The MC-30 is a basic cable that supports remotely triggering the shutter with an intermittent push button and also has the ability to manually lock the switch (and thus the shutter) open for long exposures. Pretty simple.
The MC-20, on the other hand, appears to be a high tech gizmo, sporting an LCD display and a number of buttons. But what does it do? There is a basic shutter release button as on the MC-30, but for long exposures you can set the desired shutter speed on the remote (up to 9 hours, 59 minutes, 59 seconds), then press the button. It also supports a timer mode that will count up starting when the shutter is pressed.
The MC-20 is also roughly twice as big as the MC-30 and costs nearly twice as much also. It works fine, but it really doesn't do anything you can't do yourself with the MC-30 and a wrist watch.
So, my advice is to save your money (and space in your camera bag) and stick with the MC-30 cable. Sometimes, simpler is better.
Update 12/04/2007 - These days, the choice is between the standard MC-30 remote cable and the new MC-36 multi-function remote. Just as it was with the MC-20 though, the MC-36 is still twice as big and twice as expensive as the MC-30. Save your money and your camera bag space, unless you really, really need the extra features.