Most electronic flashguns can be fired manually. Look for a "test" button. As for what is good, it all depends on how large/bright/expensive you want to go, and whether you'll always be using the camera on auto rather than selecting any manual aperture settings (if you have that option with your camera).
With my old, basic digital point-and-shoot, I often used the small, cheap slave shown
here, which worked OK for short-range photos. If you cover the slave part, you can just fire the flash manually (which I usually did). If you do get a bigger flashgun, I suggest one with adjustable intensity. That way you can dial up and down your power so you don't over-expose the close-up shorts or under-expose the larger room/passage shots - especially when you leave the camera set on an auto sunset/fireworks type mode.
If you haven't already, take a read through these threads...
Those annoying spots
Slaves for Digital Cameras
New to Digital Cameras