just about any leg exercise is good for mt. biking, but personally, i prefer core and free weights with one exception: calves.
there is really no need AT ALL to hammer your legs constantly with weights in season or out. honestly. if you do ONE exercise that is core/leg focused and one secondary exercise at least once a week you can make gains. i know this sounds wacky and completely contrary to everything you've probably ever read, but trust me, this works. like anything, the keys to success are intensity, focus and technique. if you go to the gym, focus on what you need to do, this workout can be done, start to finish in 45 minutes tops.
i would define lower body core focused exercises as any free weight movement that combines the use of multiple muscle groups, including stabilizer muscles and secondary muscle groups. dead lifts, power cleans, squats. these are the three exercises that i focused on. i worked things a little different b/c i was looking purely for strength gain as opposed to aerobic strength gain, but the philosophy would be the same. 4-8 sets depending on your existing conditioning, each set in the 8-12 range. less than 8 reps and you're building more mass than maybe you want.
the only thing i would caution against with these exercises is just jumping into them. for these multi-muscle movements, NOTHING beats experience. i can tell you from my experience, that despite spending the better part of 10 years in a gym, the last three focusing on this type of workout, my techniques are very good, but still not perfect. basically, find someone that you've seen doing these and ask them. then get a second opinion. it is a very different type of person that acutally does these types of exercises. the reason why? because they are DIFFICULT...and they HURT even when you do them right. BUT the gains you can make from them are leaps and bounds above the typical four sets each of leg extensions, leg curls, leg press, smith machine squats, calves, etc.