A great product: The Bus Pirate v.3
The Bus Pirate v.3 is a great piece of open source hardware and software bundle, built by Ian Lesnet & Co. @dangerousprototypes that can easy substitute some quick-build interfacing adapter for I2C, UART, 1-Wire, SPI, MIDI and many other serial protocols.
One important thing is that is 5.5v tolerant and supports some scriptable binary bitbang. In other words you can test at bit level various things that supports syncronous and asyncronous serial protocols.
Is built around a FT232RL converter, a PIC24 and a smart power supply (5v and 3v3 supplied to the target as software option).
Some features here: http://dangerousprototypes.com/docs/Features_overview
Unfortunately… somehow I managed to burn his first 3v3 Micrel LDO (VR2 on the schematic/pcb). A short circuit on the cable connector and the LDO is gone.
To use the BusPirate again I had to switch the 3v3 LDO’s, the first one powers the board and the second one is for the “power to target” mode. Now the board is without the 3v3 supply option.
A v3 vs. v4 comparison: http://dangerousprototypes.com/docs/Bus_Pirate_v4_vs_v3_comparison
The 3b hardware overview: http://dangerousprototypes.com/docs/Bus_Pirate_v3b
Some places to buy:
Seeed: http://www.seeedstudio.com/depot/bus-pirate-v3-assembled-p-609.html?cPath=61_68
Watterott: http://www.watterott.com/de/Dangerous-Prototypes?x76825=eb8db1605207013dc33987d199f09e3d
Adafruit: http://www.adafruit.com/products/237
Other Bus Pirate resources:
Wikipedia: http://en.wikipedia.org/wiki/Bus_Pirate
A very good Italian article about 1-wire communications and Bus Pirate: http://www.settorezero.com/wordpress/il-bus-pirate-cosa-e-come-funziona-esempio-di-comunicazione-con-bus-1-wire/
A very good I2C article: http://www.maartendamen.com/2011/04/bus-pirate-talking-to-chips-bmp085-used-as-sample/
3-wire protocol article: http://bobdasquirrel.blogspot.it/2013/01/bus-pirate-meets-93lc46b.html