There's already plenty of similar pics floating around and where to get this voltage and that, some good, some not so good, and some that haven't been done before, so I did these up and tossed them together all in one place for anyone that needs/wants them.
There are only a few Ground spots marked throughout (Black in the first pic, Blue after that), mainly for a quick spot to stick a meter or whatever, but if ya want a known ground source use the metal cage or spots under the power plug, as long as the motherboard is installed and screwed down the metal cage is ground. You can also use the shielding of any of the Main Connectors that are on the board, USB, Memory Card ports, any of them, they're all grounded and are very easy to solder to. Just be careful and don't have the heat on them too long because they're plastic inside.
This is under the Power Connector, you have access to the 12v right from the PSU (when it's on) as well as the 5v Standby voltage. The Standby voltage is always present, as long as the PSU is plugged into the 360 and a good outlet, it remains on even when the 360 is turned off. The 12v, and every 12v spot as well as other voltages, in the 360 are Switched, they turn on/off with the 360. If the 360 isn't on, there's no 12v anywhere, if there's no 12v there's no 5v, 3.3v 1.8v and whatever other voltages are in there I haven't traced out yet.
Here you can get to the 5v Standby (Red) from the top of the board, there are also a couple of 12v (Yellow) spots that are fairly easy to use as well. Remember Blue spots are Ground from here on.
Here is the older good 5v spot and is still a good place to get 5v from, but some may not like soldering onto that SMT Capacitor there. You can also use the small spot there at DB6G1. There is also a 12v spot there at the tab of the MOSFET Q6F1. Soldering onto a tab of one of these MOSFETs can be difficult if you don't have a Soldering Iron that can sustain the heat long enough to get the tab and large plane it's soldered to hot enough to flow new solder on and melt the old as well.
These are 5v spots that are under the board. They're right about the exact same spot as the 5v on the top ya just looked at, there is also a 12v spot here.
Here are some other 5v locations (The ones in Red) that are at the front left of the 360. The Violet spots are 3.3v for the DVD drive (Large Regulator in the middle U1F1). 3.3v for the Memory Cards (Small Regulator at the bottom U1F2) and the 3.3v Regulator at the top (U1E1) supplies voltage to a 1.8v Regulator that's under the board (U2T1) that powers the Southbridge. There's also a 12v spot there at the tab of Q3F1.
The row of MOSFETs on the right hand side of the 360 (The pic is turned) all have 12v on the tabs.
These are all Standby voltages, Red = 5v, Violet = 3.3v and the pink Grapefruit looking color is 1.8v, these voltages are always present as long as the 360 has power, doesn't matter if it's turned on or off. The 5v is the same as the 5v Standby that's the Power Connector. The 3.3v powers the RF board and I'm sure a few other places and the 1.8v goes mainly to the Southbridge, I haven't finished tracking down all the places they go to yet.