I've noticed a lot of prejudice towards single leg compound exercise. Variations of the squat and things like lunges, Bulgarian Split Squats, and step ups are frequently labeled sissy or ineffective. IMO, if you really think that then you probably haven't done them.
There are so many advantages to them. One of the major ones is that you can effectively target certain areas of the leg with a heavy functional movement. So that you can bring it up in size in a way that directly influences you primary lift like squat or deadlift. So many people waste their time on leg extension, which, while as an accessory may add size to the quads they will hardly do shit for influencing your squatting ability or adding size to your entire body. But you can prioritize quads or hams very easily with subtle position changes with single leg variations with the advantage of actually doing an effective squat variation instead of an "extra" isolation.
Another great advantage is in the area of injury. If you are getting over a back or knee injury then there are many reasons why these are a good way to go. For one thing the load on the back is reduced a great deal. Also, the forward lean needed is reduced so extensor forces on the back are greatly reduced. Similar reasons for the knees. This is harder to explain but it's basically the mechanics of it which make it very difficult to overload the knee without using a weight that is beyond the hip flexors ability. With the bulgarian split squats this is pretty much impossible. BTW, if I refer to split squats I mean bulgarian split (back leg elevated) squats and NOT lunges, although they're ok too.
The first time you do bugarian splits with heavy ass dumbells hanging from your hands you will probably have to force yourself to do the second set. Don't knock this stuff until you try it.
On to the purpose of this. Here is an article by Mike Robertson (from T-Nation, sorry can't help it) that has a lot of variations and explanations. It has a bunch of images so I'll just link it.
Single Leg Supplements
Single Leg Stuff
Threaded Mode