You can also browser the Tahoe Area Map to see where the trails are.
Hole in the Ground - A great Sierra ride right off of Highway 80 on Donner Pass. This trail has a lot of challenging terrain, fast rocky singletrack, granite playgrounds, and some great views of the Sierra Nevada Mountains.
Mt. Rose to Kingsbury Grade - Mt. Rose to Kingsbury Grade (Tahoe Rim Trail + Flume Trail)
Northstar at Tahoe - During the summer months they open up the ski slopes at Northstar (and many other ski resorts) for mountain bikes. While I usually try to earn all of my downhill by climbing this is a great oportunity to get some concentrated downhill skills practice.
Silver Lake - Horse Canyon - An incredible - must ride - trail right off of Highway 88 near Tahoe. This trail has some incredible Sierra Nevada riding, awesome views and the downhill at the end is sublime. This trail is nowhere near as popular or well known as many of the other Sierra Nevada rides such as Hole in the Ground or Mr. Toad's Wild Ride but it is every bit as good.
Tahoe Rim Trail - The Tahoe Rim Trail is a huge system that surrounds the entire Lake Tahoe Basin. Almost all the other rides in the basin touch the Rim trail in one way or the other. Much of this trail is rugged challenging singletrack which is suitable for advanced riders only.
Tahoe Flume Trail - One of the most popular mountain biking trails in the area due to beautiful scenery and an easy trail
Mr. Toad's Wild Ride - Located off Highway 50 just inside of California, Toad's Wild Ride is a Tahoe area classic. This is an awesome trail with all the elements that make a great downhill run.