I find this useful when doing storyboards and I have a bunch of psds I want to convert into jpgs without doing it one by one via Save As..., which carpal tunnel would more than love. Let photoshop do it for you. I'm using Photoshop CS3 in this case, so it may look different, or not.
Go to File > Scripts > Image Processor... > in the pop up menu, this is where you can do it a couple different ways. Follow Step 1 to get your files, either you have them already open, or Select Folder to find them, or use Adobe Bridge beforehand and use Image Processor through Bridge.
Follow Step 2 to choose the location of where to save all of the jpgs. Normally, I choose Save in Same Location, because this automatically creates a folder called JPEG and spits it all out in there.
In Step 3, choose JPEG and quality level. > Hit Run, and just let it run.