Fitbit is a line of health/fitness products which use various sorts of network connections to exchange data. There are several models of pedometers, which track the user's steps and other activity (such as climbing stairs) and sync it either to a PC (with an attached dongle) or a smartphone (via Bluetooth). The computer or phone in turn connects to Fitbit's website to store the data on their server. Fitbit also makes the Aria scale, which sends weight readings through the net by WiFi. (Configuring that device requires setting it up as a wireless router and connecting a computer or phone to it temporarily instead of your regular router in order to access its setup screen as a web page.)
Some models also track your activity while sleeping (by putting the device on an armband), giving statistics on how well you slept (presumably by noting when you're tossing and turning versus remaining still).
Exporting your own personal data from the service is, unfortunately, considered a "premium" service you have to pay extra for. This supports CSV or XLS formats, but still doesn't give you the full extent of the data being kept from your devices. All sorts of minute-by-minute data is logged, but only daily totals are given in the exports.
There is also an API for outside developers to access Fitbit data.
A doggie-style device of a similar nature for "Man's Best Friend" is Whistle.