For those who wear a heart rate monitor:
My riding setup is probably weird by the practice of the rest of you. I control my effort and thus my speed by my respiration rate, not by cadence or anything else. So I wear a heart rate belt all the time. The latest is a Bluetooth low energy wireless sender belt called the Polar H7 (fabulous -- the reviewers who didn't like it either got a dud or didn't RTFM), which works with the iPhone 4S and 5, and possibly some Android devices too. You can get all kinds of free and paid software to work with it. Many of these pieces of software link up to the GPS and/or to Google Maps with or without the GPS enabled. In fact you don't need to wear the belt to see the map (well, as well as you see it in sunshine on the phone screen -- the visibility in any kind of bright light is the most disappointing feature of the iPhone). The software I use is Polar's own Polar Beat. It doesn't appear to have any of the map features already discussed above, it just shows you a map of your surroundings, without finding a path through it for you. Additional, paid software from Polar offers more exercise analysis features and social linking, but doesn't appear to offer more in the line of maps. A feature on the free version that I do use is the facility to show you where you've been, and to count layers of circuits if, for instance, you're riding intervals on a short course, as I sometimes do.
Topeak also has a Bluetooth 4.000 sensor and software, which is a bit more attractive, but the Toepeak software doesn't work with the Polar sensor.
The Polar H7 sender and belt was about 63 Euro landed here, the software was free, the phone I already owned, so not a cheap setup, but it gives you HRM. GPS, a relatively accurate but very basic bike computer, plus basic exercise analysis tools. The belt works better than the Sigma I had before that served for years (the Sigma kit including the belt, a well-styled watch and a superior bike mounting, cost then and still costs about 40 euro, and gives you far more detailed analysis tools...)