There are many heart rate monitors available in the market. Basically accuracy of HRM depends on its working principle. Some of the accurate and popular HRM are Polar FT60, Polar FT1, Timex Ironman Race Trainer, etc. This is all about HRM.
I can suggest one more innovative thing for MTB users, One can mount bicycle racks, if they want to take it as long distance for adventure or something. Here is a site: http://www.birack.com, which will help you to select different types of rack for your bicycle.