Some consider this the best ride in Montana. There is a stiff, 2k ft climb to a gorgeous high alpine lake. Ofcourse, this makes for a killer downhill on the return. You can continue beyond Emerald Lake to Heather lake for an additional mile, but the best of the singletrack is already behind you at this point. This is a popular trail with hikers and equestrians, so going on a weekday is best (all those other trail users will break your flow on the plummet back to your vehicle). The bottom of the \"singletrack\" is pretty wide. Only this, plus the relatively short length (<5mi each way) keep this from being a 5-star in my book.
From Bozeman, drive south on 19th, turn left on Hyalite Canyon Road, follow this past the reservoir, go left at the split, and continue on until the road ends at the trailhead.
http://www.singletracks.com/item.php?c=1&i=2314