First, I have to agree and echo Catch's statement. Data is most vulnerable whilst in transit. The most popular and efficient means of protecting data in transit is encryption.

Secondly, I agree with Maestr0- Antisniff is a decent application which will work as intended.

From the overall context of this thread and the original question, you [jojo...] may be putting the cart before the horse imho. If you're truly worried about someone sniffing on your network, then implement safeguards before it happens rather than worrying about catching it after-the-fact. Doing the former instead of the latter will not only teach you more about information assurance but also be much more productive in both the short and long term.

Cheers,
<0