Open fitness trackers

The solutions for open access to realtime fitness tracker data are limited at the moment. Getting delayed data (possibly via websites) is slightly easier.

The reselling data business model
The business models for a number of companies seems to be explicitly *not* providing you with useful data. Perhaps so they can resell data for you, perhaps for a consistency experience, perhaps to maintain brands.

Bluetooth  wrist bands
tl;dr Some work has been done on reverse engineering the xioma band, it's plausible that you could finish this off.

My current understanding is that the fitbit has not been reversed engineered effectively and using encryption (https://pewpewthespells.com/blog/fitbit_re.html). There are a couple of low cost ble wrist bands. THE ID107 and xiomi mi come to mind. I did not have success pairing to the ID107 band.

I could however pair with the xiomi mi band, but kind of stopped there and didn't get any further. Looking now it seems like there's some open source code to reverse engineer it: https://bitbucket.org/OscarAcena/mibanda, but no one seems to have actually been able to read the heart rate data. There's a fun comment here that says, "it's easy just google it": (http://allmydroids.blogspot.co.uk/2014/12/xiaomi-mi-band-ble-protocol-reverse.html), but googling the terms doesn't really say anything useful.

See this point for a jumping off point for reverse engineering heart rate information: http://stackoverflow.com/questions/37779288/heart-rate-measuring-using-xiaomi-miband-1s-and-ble. I imagine the world would be grateful if you finished off this reverse engineering as it would open the door for lots of apps for a fairly cheap heart rate monitor.

Angel Sensor
Angel sensor was a kickstarter (I believe) to provide a properly open API, it has produced two versions of a band. However, you don't actually seem to be able to buy them from anywhere, no one is even selling them on amazon for 7 times their original prices. I imagine if you know there right people you might be able to get hold of one though.

Bluetooth chest bands
There are a number of BLE chestbands. I get the impression that they might be easier to interface with, because I found generic android apps for communicating with ble heartrate monitors that listed some of these bands. Polar are one of the companies that provide these.

Ant+
Ant+ is a protocol similar to bluetooth that seems to be related to the garmin company. I think that it's slightly more open that other standards, as I get the impression is garmin was attempting to create a hardware ecosystem around its other products. I think it gets used a lot for cycle computers. I get the impression "dumb" devices often talk ant

You can buy inexpensive ant+ dongles that work with linux. Check out this post: https://johannesbader.ch/2014/06/track-your-heartrate-on-raspberry-pi-with-ant/

Also there is a long standing open source program called "Golden Cheetah" that is used for real time monitoring cycling power monitors. It doesn't seem to work with heart rates though.

There seem to be some ant heart rate monitors though: https://www.dcrainmaker.com/2014/01/mio-link-first-look.html.