The Flex Timer app can integrate with any heart rate monitor that supports the bluetooth heart rate profile. This open standard allows any app to integrate with any heart rate monitor without tying you in to a vendor specific application.
Unfortunately, some companies do not follow this standard and instead use a proprietary protocol so that only their apps or their approved partner's apps can integrate with their technology.
The following cheststrap heart rate monitors will work with the Flex Timer:
The following wristband/armband heart rate monitors will work with the Flex Timer: