If you plan to spend a lot of time down low working with these protocols, definitely get a Saleae Logic. I can't recommend it enough:
It's a small USB-powered digital logic analyzer in various sizes with the buffer size limited only by your host computer's memory. The real magic is their software - it gives you protocol analysis capabilities that used to cost you thousands if you were an HP/Agilent/Tek customer. You get I2C, SPI, RS232/485, DMX, 1Wire, CEC, USB, tons more. Best part? The host software also runs on Mac and Linux.
And if you are interested, support these guys and buy the real thing. Don't buy a knockoff and then use Saleae's software (which they give away for free to let you try).