Via IronGeek, an awesome list of tools and videos that will help you do sniffing and MITM attacks.

I link to the videos in Vimeo, but his site is really worthy :)

Sniffers Class Part 1 from Adrian Crenshaw on Vimeo.

Sniffers Class Part 2 from Adrian Crenshaw on Vimeo.

Sniffers Class Part 3 from Adrian Crenshaw on Vimeo.