Methods to test/debug I2C protocols?
To me, robust means: throwaway price, comes with schematics and software and I could build or fix it at home.

Here are two favourites, both around the $20 mark, exposing USB serial and PC scriptable:

The Bus Pirate is a universal bus interface that talks to most chips from a PC serial terminal. Lots of serial protocols are supported: I2C, SPI, 1-Wire, JTAG etc. as well as a bit of AVR/PIC programming and EEPROM dumping. It has a low speed logic analyser and sniffers for I2C and SPI. There's an active forum with good support.

(It's a PIC24 with a USB serial port and some bus switches for open-drain pullup trickery. Windows only MPLAB needed for firmware development. ds30 serial bootloader pre-installed for "field upgrade".)

USerial is an I2C, SPI and GPIO bridge, using an Atmel AVR. It'll also run on any AT90USBxxx board, eg. Teensy, Benito, Nanduino.
Compiles with gcc. Well documented.