1. PocketNES, hands down. See http://www.pocketnes.org for the official 9.98 release, or visit http://www.pocketheaven.com/boards/viewtopic.php?t=2147 for the unofficial 9.98 release that has more features. 9.99 was also released, available at http://www.pocketheaven.com/boards/viewtopic.php?t=4709 but there have been reports of some incompatibilities with it, so I personally use 9.98.
2. I don't know if it exists
3. The GBA isn't powerful enough to emulate the Genesis. There is a limited emulator for the DS called FrodoDS
4. Goomba Color. The latest official release is Apha 6, but you can download an updated WIP release from http://www.dwedit.org/dwedit_board/v...c.php?pid=1740 which includes a fixed black border (instead of the ever-changing border from Alpha 6) and limited Super Gameboy borders and palettes. There's also a GBC + SGB mode that sometimes will allow you to use both the GBC colors and the Super Gameboy borders, but it doesn't work on all games with support for both.
There is no mp3 player, as the GBA is not powerful enough to decode mp3s in real time. However, there are two music player applications you can use. I already posted a huge summary of each of them at http://www.emuboards.com/invision/in...howtopic=27935 so go there for info. Long story short, I recommend the GSM Player.