From the ATX Specification, Version 2.2

Main power connector, female-ish - metal sleeves within plastic pins (PSU side): looking at the plug, facing the pins with tab side up, pins 1-12 go left to right on the bottom row, and 13-24 go left to right on the top row. (XXX - spec seems to say this is for the "board mounted connector" ie male; which is which?)

The 20-pin variant just omits 11, 12, 23, 24.

1  +3.3V  13  +3.3V
2  +3.3V  14   -12V
3    COM  15    COM
4    +5V  16 PS_ON#
5    COM  17    COM
6    +5V  18    COM -+
7    COM  19    COM -+
8 PWR_ON  20     NC [formerly -5V?]
9  +5VSB  21    +5V
10 +12V1  22    +5V
11 +12V1  23    +5V
12 +3.3V  24    COM

+12V power connector (CPU): 1-2 and 3-4 numbered in like manner.

1 COM  3 +12V -+
2 COM  4 +12V -+

Peripheral power connector ("Molex"), female (PSU side): facing the plug with the side of the housing with truncated corners up, sleeves 1-4 go left to right.

_ 1 +12VDC \
  2    COM |
  3    COM |
  4  +5VDC /

Serial ATA (SATA) power connector: pins 1-15 count from the notched/keyed end.

+-------+
|       |
+--+    |
   | 1  | +3.3V
   | 2  | +3.3V
   | 3  | +3.3V
   | 4  |   COM
   | 5  |   COM
   | 6  |   COM
   | 7  |   +5V
   | 8  |   +5V
   | 9  |   +5V
   | 10 |   COM
   | 11 |   COM
   | 12 |   COM
   | 13 |  +12V
   | 14 |  +12V
   | 15 |  +12V
   +----+

Voltage tolerances: +-5% for all positive rails and +-10% for all negative rails (per Table 6).

Control signals:

PS_ON# : active low, turns on the main power rails; when pulled high or left open, power rails should be held at ground.
5VSB : standby voltage present even when main rails are powered down (for soft power control / wake on LAN); recommended to support 2A.
PWR_OK : asserted high by PSU when the +5 and +3.3 V outputs are above undervoltage thresholds and sufficient stored energy is available to continue operation within spec.

Typical color code, though it doesn't seem to be stated in the spec:

 COM black
3.3V orange
  5V red
 12V yellow