Companies like Network Optics make incredible taps, for all sorts of media, but if you have 10/100 home network then for $18 in parts from home depot you can make a tap and send the output to YAF/snort/tcpdump/wireshark and see if any data is leaking that should not be.
I have been doing Flow Analysis lately instead of using other tools. I like YAF . Then again I work on it...
If you want to see step by step instructions on setting up a flow collection infrastructure look at this wiki page.
Step 1: Parts
3x Leviton Multi Use Cat 5e Jacks (5G108-W)
- I used 2 white and 1 blue, to let me know which one is the tap.
Leviton 3 port wall plate (#41080-3W)
5 inches of cat 5 cable