A number of people have asked about L-Carnitine. Here is what I know to date. Carnitines come in a variety of compounds that are commercially available as L-Carnitine, Acetyl L-Carnitine, Propionyl L- Carnitine (they are all pricey). Although I have found no large scale studies there are several small scale studies at respected institutions where carnitines have been effective for such things as cancer related fatigue, male age related dysfunctions (e.g. fatigue and erectile dysfunction), alzheimers, diabetic neuropathy, and a host of other maladies. I took L-Carnitine for an extended period for its reputed ability (again small studies) to reduce lipoprotein(a) by 8-10%. It did not work for me but again, I am a statistical population of one and no inferences can be made from my personal experience.
Wikipedia has a decent technical description of carnitines here . There is also an old Medscape Article that pretty much puts carnitine in the "so-so" class of nutraceuticals. The evidence is positive but extremely weak for heart benefits. It is fairly benign in terms of side effects and toxicity at levels at or below 3 grams per day based on the reports I have seen.