The usual response to a downed tree across a trail is to cut it away, returning the trail to its original status. Yet, often a downed log can add character and challenge to a trail. A log-pile ramp can be a fun addition to your favorite singletrack, and is certainly a better option than a detour around the obstacle that widens the trail. The key is to recognize when a log-pile ramp is appropriate versus when it would be better to clear it away.
Log-pile ramps are great as long as they are on more advanced trails and are consistent with the flow of the trail. On tight, technical singletrack, ramps fit well with the chess-match-like feel of the trail. Conversely, on a wide open, smooth singletrack, a ramp wouldn't flow, and could in fact be dangerous. Log-pile ramps should not be located on trails frequented by beginners, or close to the trailhead where novices are more likely to ride.
A few tips on building log-pile ramps: pile smaller logs parallel to the big log. Big rocks also work well. It's a good idea to put pegs in the ground to keep the outermost log from rolling out from under the pile.
In the no-brainer category, don't cut live trees to create log-pile ramps - use deadfall. Finally, get permission from the land manager before proceeding with any trail construction or maintenance.
For more trail tips and information visit IMBA.