For cross country ski rentals, lessons, and 28 miles of groomed trails at the foothill of the
Mammoth Crest, check out Tamarack Lodge.
Tamarack Lodge is located at Twin Lakes at the end of Lake Mary Road 
(the road is closed past this area) and offers 28 miles of freshly groomed track with skating lanes that
winds around lakes and has astonishingly beautiful 
views of the surrounding mountains. 
You do need to purchase a trail pass to use these trails.
Tamarack also offers lessons, both group and private from beginner to advanced.  

For those who enjoy taking to the trails on your own, there are 19 miles of maintained but non groomed trails through out Mammoth Lakes.  These are marked by blue diamonds and are listed below. 
Free maps can be picked up at the visitors center or ranger station.

Blue Diamond Trails:

Shady Rest Trails
     Type: Beginner
     Length:  5 miles loop
     Location:  Off of HWY 203 just before the Visitors Center as you head out of town.  It is the  
     next left turn after Old Mammoth Rd.
There are three main loops with all of them beginning at the Shady Rest winter Trailhead (you will know because the road will cease to be cleared of snow).  These trails are frequently skied, snow shoed, and hiked as they are very convenient to town and provide a very pleasant jaunt through Jeffrey Pines.

Knolls Trail
     Type:  Intermediate
     Length:  5 miles one way
     Location:  This trail can begin at two different locations:  1.  Mammoth Scenic Loop 1.5 miles 
     North of Hwy 203. This eliminates most of the elevation gain but has a intermediate descent. 
     2.  Shady Rest  'C' loop from the park just past Old Mammoth Rd.  This way has a ton of 
     climbing but a beginner in fairly good shape will have no problem.  
This trail crosses the Mammoth Knolls and winds through a Jeffrey Pine and Lodgepole forest with views of the Minarets and Sherwins.  There are also two side trails that you can enjoy.  1/5 mile Mammoth Town Overlook trail and the 1 mile Clyde Minaret Vista Trail.

Scenic Loop Trail
      Type:  Intermediate to advanced
      Length:  2 mile one way
      Location:  Mammoth Scenic Loop across from the Knolls trail.
This short trail joins the Inyo Craters Trail 1/2 mile from the Inyo Craters trailhead.  It contains two short but steep descents.

Inyo Craters Trail
      Type:  Beginner to Intermediate
      Length:  1.3 miles one way
      Location:  Mammoth Scenic Loop
This trail provides access to the Inyo Craters, two small volcanic craters with lakes at the bottom.  The first 2/3 of the trail is flat and great for beginners but the last portion contains some short but very steep climbs which means steep descents.

Earthquake Fault
      Type:  Intermediate to Advanced
      Length:  4.5 miles one way
      Location:  Off of Minaret Rd on the way to the Main Lodge of Mammoth Mountain you will 
      see the parking lot on the right hand side.  There is limited parking in the winter in a pull out 
      slightly down and on the other side from the fault park.  
The trail ascends to a saddle north of the Earthquake Fault and then descends into the Dry creek drainage then meets the Inyo Craters trail.  There are a number of steep, narrow descents where strong intermediate or advanced skills will be required to navigate the deep snow.  60% of this trail follows the orange diamond snowmobile trails so be aware.