I've gone ahead and recompiled these, should be the most up-to-date packages. This is the basic tool of GBAFE hacking; it edits the game's raw hex code, but puts the jumble of values into a labeled interface for ease-of-editing.
And then, of course, a plug for Hextator's Nightmare 2.0 [broken link].
As of 2009/03/28/18:27, this application can parse all the component types that the original Nightmare can.
Because this application is compatible with FEditor Adv, avid FE hackers may want to consider switching from Nightmare to Nightmare 2.0...NOW.
I decided to go ahead and release what I have.
It has support for checksum validation and ID string validation.
It has the ability to use base pointers instead of just using base addresses and takes advantage of this to expand data.
Also, the doc is minimal (actually, you could say I haven't started it yet, despite there being some content in the doc folder), so you'll have to look at the source code if you want to learn how to use those new features and can't understand the example below (or you can bug me on AIM/MSN).
The coolest part is that it tells you the exact address you're editing down to the first bit of the data next to each editing control.
That alone, plus the fact that it's multi platform should get you to start using this now and then. ;D