I just came back from an awesome day at our partner machine shop, Birdwell Machine (http://www.birdwellmachine.com/)!
Anyway, since I’m a programmer, I was naturally curious about the G-code used on the CNC machines. So, I’m looking for some general tutorials or know-how. Any ideas?
HAAS Has a really good manual on it and a quick reference card. Also, there is CNC Machining magazine which is really cool. If you have any basic questions on CNC operation or programming, let me know and I’ll try to help.
NC Codes
-Block Number (N)
-Preparatory Codes (G)
-Miscellaneous Codes (M)
-Priamary X Motion (X)
-Priamary Y Motion (Y)
-Priamary Z Motion (Z)
-For Circular interpolations— I=X J=Y and K=Z
-Feed rate (F)
-Spindle speed (S)
-Tool Type (T)
Preparatory Codes:
G90 - Absolute Coordinates
G91 - Relative Coordinates
G00 - Rapid Traverse (non-cutting move)
G01 - Straight Line Interpolation (cutting move
G02 - Circle Interpolation (cw)
G03 - Circle Interpolation (ccw)
G04 - Dwell (wait) Pause between motions on all axes.Time in seconds -
example: G04F2 -pause for 2 sec.
G05 - Pause - waits for user intervention
M Codes - Miscellaneous
M00 - Pause
M01 - Optional stop
M02 - End of Program
M03 - Spindle on
M05 - Spindle off
M06 - Tool Change
M08 / M09 - Accessory # 1 on / off
M10 / M11 - Accessory # 2 on / off
I hope that this has helped a little bit…here’s some sample code…
N00 G90 ;
N01 M06T1 ;
N02 M03 S3000 ;
N03 G00 Z .1;
N04 G00 X.5Y.5 ;
N05 G01 Z-.0625F9 ;
N06 G01 X.5Y1.5;
N07 G01 X.875Y1;
N08 G01 X1.25Y1.5;
N09 G01 X1.25Y.5;
N10 G01 Z.1;
N11 G00 X1.75Y1.5;
N12 G01 Z-.0625F9;
N13 G01 X1.75Y.5;
N14 G03 X1.75Y1.5I1.75J1;
N15 G01Z.1;
N16 M05;
N17 M06T00;
N18 G00X4Y3Z3;
N19 M02;
As an excercise, it helps you to learn by writing in what each line of code means to the right of the semicolon. 
G-Code is one of my favorite programming languages! I learned it from a manual that came with the CNC machine that I used, but I found the following web site which looks like a good overview:
http://www.linuxcnc.org/handbook/gcode/g-code.html
Good luck! By the way, it is rather uncommon to write G-Code by hand. Lots of CAD programs are available with manufacturing packages as well, and these manufacturing packages can generate G-Code for your CAD drawing automatically. You simply have to tell it which surfaces you want to machine with what tools, how your stock is mounted, specify some parameters (such as feedrates, spindle speeds, and tool properties), and it will generate the G-Code automatically for you. In any case, it is still pretty important to know the main G-Code commands if you are going to operate a CNC machine.
And here is some of the G-Code used to machine an omni-wheel. This is G-Code that I generated automatically from I-DEAS:
(OPERATION CATEGORY & TYPE: Milling Volume Clear)
M05
(TOOL IDENTIFIER: End Mill 01/16)
(POST TOOL ID: 0)
(TOOL DESCRIPTION: 1/16 End Mill (2 flute))
(TOOL STATION NUMBER: 11)
(Start Region 1 – Depth: 0.304375)
T11 M06
M01
G56 H11 D01 Z.72
M08
S8000. M03
G00 X1.7454 Y1.1004
Z.42
G01 Z.3044 F7.5
X1.7385 Y1.099
X1.7527 Y1.0322
G03 X1.757 Y1.0271 I.0069 J.0015
G01 X1.7594 Y1.025
X1.7601 Y1.0238
X1.7621 Y1.0264
X1.7653 Y1.0294
G03 X1.7665 Y1.0351 I-.0057 J.0043
G01 X1.7523 Y1.1019
X1.7454 Y1.1004
Z.4044
G00 Z.72
Can you guess which part of the omniwheel this is for??? 
Interestingly, CNC machines used to read programs off of a roll of tape (holes were punched out in rows and columns to encode the commands). The above program, if I had pasted the whole thing, would require a roll of tape over one mile long in distance!!