I've been playing with hardware now for a year or so and this may be a good time to summarize some of the interesting products I've found. I confess that most of the real product-making has been done by my colleagues in Beijing, but I try to stay on top of it by playing as much as I can, and here are some things I've learned.
First, for sources of good up-to-date information, it's hard to beat the hackthings blog. Most every day, they feature an interesting new product or idea, often with background of how it was (or will be) manufactured or sold. Whether you're a hobbyist or a potential startup, you'll also appreciate the lengthy list of hardware resources/tools on Steve Blank's entrepreneur site.
If you're going to teach yourself hardware, Arduino is a great place to start (as I did), but you can quickly burn through a lot of parts. Instead, use an Arduino software simulator like the one from 123d.circuits.io. For a similar browser-based simulator for non-Arduino purposes, try https://www.circuitlab.com/.
If you're building self-tracking body hardware, you might check out the $500 E-Health Sensor Shield Kit for Arduino and Raspberry Pi. It comes with ten body sensors, including pulse/oxygen (SPO2), body temperature, glucometer, galvanic skin response, EMG, ECG and more. For instructions on how to just make it all from scratch, see this DIY EEG/EKG/EMG site.
Low-power devices connected to your smartphone will use Bluetooth Low Energy (BLE), and people have suggested I check out the $35 BLE Mini from Redbear Labs, the $100 nRF8001 Development Kit from Nordic Semiconductor, or the $30 LightBlue Cortado (shipping in mid-2014).
I have a ton more links in a private Evernote folder, as well as on my public Pinboard account. As always, ping me if you have other suggestions.