Following is a little description of some of the tools.
Other two copiers are designed by P. Felber and Yvan Rivard. Pascal's version is more advanced than Yvan's, but the functions are basicallly the same. Both copiers plug into a Centronix parallel port and allow downloading the cartridge code into files.
Logical analyzers allowing to trace various signals in GameBoy circuitry are also very useful for hacking. Jeff Frohwein came up with a circuit of a GameBoy-oriented logical analyzer, as well as with a 4MBit EPROM emulator allowing to download and run the code on a real GameBoy. You can get Jeff's designs off his WWW page, or off the Hardware Info page on this site.
Another assembler is written by Jeff Frohwein. It is based on TASM, a retargetable assembler package, distributed on the shareware basis.
GBSim by Jens Restemeier is another emulator which can be very useful for debugging. It is an MSDOS-only application, written in 80x86 assembly. As result, it is quite fast, and has an excellent builtin debugger. Emulation is less precise than in VGB though.