Thanks to @johnnygal @Kato and @mholgatem for compiling and contributing to this!

MAME Rom Sets
MAME4ALL uses 0.37b5
AdvMAME uses 0.106

Properly Verify and Name Roms

http://pimame.org/forum/discussion/1083/how-to-properly-verify-and-rename-roms#Item_1

Rotate Display
type:

sudo nano /boot/config.txt

Then you need to add a line at the end that reads ONE OF THE FOLLOWING:

display_rotate=0        Normal
display_rotate=1        90 degrees
display_rotate=2        180 degrees
display_rotate=3        270 degrees
display_rotate=0x10000  horizontal flip
display_rotate=0x20000  vertical flip

Note: the 90 and 270 degrees rotation options require additional memory on GPU, so won't work with the 16M GPU split.

Problems with HDMI
If there is no video using a HDMI to DVI lead or adapter, edit /boot/config.txt and uncomment

hdmi_force_hotplug=1

If no analog audio, try editing config.txt and uncomment

hdmi_drive=2

USB Soundcard Setup
To use a USB soundcard:
Edit /etc/modprobe.d/alsa-base.conf:
and comment out the lines below:

# Keep snd-usb-audio from beeing loaded as first soundcard
#options snd-usb-audio index=-2

You can check the GPU's firmware version by entering the following:

sudo /opt/vc/bin/vcgencmd version

Shutdown Switch
To install python for shutdown switch:

sudo apt-get install python-dev
wget http://pypi.python.org/packages/source/R/RPi.GPIO/RPi.GPIO-0.4.1a.tar.gz
tar xzf RPi.GPIO-0.4.1a.tar.gz
sudo python setup.py install

SET GPIO#17 to be internally pulled up, then need no resistors and only 2 wires
Connect one side of button to gnd and other to GPIO#17 - i.e. shorts pins 9 and 11.
Script to monitor GPIO #17:
[Save as /home/pi/bin/shutdown.py]

import RPi.GPIO as GPIO
import time
import os
GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN,pull_up_down=GPIO.PUD_UP)
while True:
    print GPIO.input(17)    
    if(GPIO.input(17) == False):
        os.system("sudo shutdown -h now")
        break
    time.sleep(1)

To set it to run on boot just add the following text to /etc/rc.local, just before exit 0

python /home/pi/bin/shutdown.py &

Increase Volume of Soundcard

amixer controls

To find id of volume device

amixer cget numid=1

Shows min/max values and current value.

amixer cset numid=1 0 

e.g. sets value to 0 (was min -10239,max 400)

EASY ACCESS WITH STATIC IP
See:

http://goo.gl/oB0dzD

Audio Related Problems

http://pimame.org/forum/discussion/769/no-audio

Run a ROM on Startup

http://goo.gl/7dwYF6

Upload Multiple Files of Folders

http://goo.gl/nZ1viJ

Two XBOX controllers in PiSNES

http://goo.gl/5UgtPV

A huge thanks to @Kato for this!


Enjoy! Hope this answers some questions!
This was updated on 3/5/14 to be easier to read and adding new items.
And then again on 3/11/14 with some new stuff and clean up.