Cause I've searched a lot for a project like this, and I haven't find anything around the Web, I would share my experience of my personal USB Keylogger.
It's not really a pure "USB" Keylogger (cause USB HID protocol is much more difficult than PS/2 protocol), but it adapt an USB Keyboard to PS/2 port, while (of course) recording the keys pressed.
In this way, even if it's discovered by anybody, it should be confused with a normal PS/2 Adapter.
That's the final result:
It need just a few component (SOIC PIC and EEPROM can be freely ordered as a sample from http://www.microchip.com/samples/ ):
-PIC 12F1822 (SOIC Version)
-EEPROM 24XX1025 (Any 1Mb version will be ok) (SOIC Version)
-2 * 4k7 Resistor 1/8 W (Or any resistor of the same value as small as you can solder in the adaptor)
-Pickit 2 / 3 (For programming the pic and reading the eeprom)
-An USB to PS/2 Adapter that can be opened.
And, very important:
-A GOOD solder and VERY GOOD soldering abilities.
EDIT (in reply to comments):
Some weeks ago, a friend of mine asked me to help him, cause his computer seamed blocked, indeed, he couldn't either type his password to login with his account..
I know that he is almost a novice and couldn't just change the keyboard instead of asking me help, anyway, i changed his keyboard, but I wanted to know why it wasn't working, I tried it on another computer without any result, except that the keyboard's leds blinked, so, I wanted to go deeper; I take a Logic Analyzer and i have analyzed the PS/2 protocol, I have discovered that the keyboard was switching off whenever the PC sends 0xFF (Reset) command.. I then solved it simply replacing a capacitor that has blown up.
This is WHY I build it (for any similar case), I'm NOT responsible of ANY aim you use this Keylogger.
Illegal is the WAY you use a Keylogger, not the Keylogger by itself.
I didn't want to make anything "immoral" or "illegal"
Step 1: Circuit
This is the complete circuit of the Keylogger, it can seems complex, but in real, it isn't.. we'll see why..
It is divided into two "zones":
One is the Adapter, which connect GND and VCC pin of PS/2 connector with their respective pins on the USB Connector and CLK and DAT pin to, respectively, D+ and D- pins.
The other zone is composed by the PIC and the EEPROM (and the I2C required pull-up resistors). The PIC take care of detecting incoming Data from PS/2, decode, and then writing it to the EEPROM.