BCAAs

As you probably know, amino acids are the building blocks of protein, so essentially they are the building blocks of muscle.  A lot of research has been done specifically on BCAAs (isoleucine, leucine, and valine) and have shown that these essential amino acids help prevent fatigue and maintain muscle muscle and strength during hard training.

BCAAs play an important role in promoting protein synthesis in a few different ways: 1. A high concentration of amino acids at the muscle stimulates protein synthesis 2. Insulin, an anabolic hormone, stimulates protein synthesis in the presence of amino acids and, 3. Leucine directly stimulates protein synthesis. BCAAs can prevent protein breakdown as well, reducing muscle soreness. They also can provide extra energy during heavy lifting, and even may help you mentally fight through a hard workout.  These benefits can result in greater strength gains.  BCAAs can be used as fuel by producing energy during a long session of intense exercise as well (for example, for endurance athletes).

There are tons of different studies out there that go into specific detail about how exactly BCAAs are beneficial.  This website is very informational if you are looking for specifics about the benefits: http://www.charlespoliquin.com/ArticlesMultimedia/Articles/Article/791/The_Benefits_of_BCAAs.aspx

Basically, anyone from bodybuilders, to regular weight lifters, to runners can benefit from taking BCAAs!  BCAAs can be taken at any time during the day, but the most beneficial times are just before or after training, and even during your workout.  They are quickly absorbed into the bloodstream.  Most studies have recommended taking 10-15 grams before your workout, another 10-15 grams during, and then a final 10-15 grams as soon as you finish working out for the best results. 

I'm currently taking USP Labs Modern BCAAs.  I've also heard good things about SciVation Xtend, but there are a ton of different options out there.