Not sure if this has already been discussed or not but I couldn't find the information when I looked for it so I thought I'd post my findings for others.

I personally would like to be able to use the maemo file browser to browse my selection of roms and simply select a rom and have it open the appropriate application. To do this is very simple. You have to have root access in terminal (install rootsh).

Open a terminal and type

We are going to edit defaults.list

The following step is optional to make a backup file
Open defaults.list with your favorite text editor and add the following lines at the end of the file:

Save and close the file. The first line associates .nes files with Ines and the 2nd line associates .gba files with VGBA. I didnt add a line for the snes emulator because when I tested it, the emulator would launch but would not start the game.

You can now click on an nes or gba rom from the maemo file browser and it will launch the emulator and start the game.

I assume you could add similar lines for the GB and MasterGear emulators but I dont have those installed so I cant give instructions for them.

Hope someone finds this useful