Active hypoxic training is a drug-free alternative to accelerating endurance performance. Essentially, you simulate training at different altitudes by controlling the density your oxygen intake. The training method has recently been the focus of a lot of attention as the World Anti-Doping Agency has decided not to ban the practice.
From Giz Magazine:
The Executive Committee of the World Anti-Doping Agency (WADA) has decided not to ban Hypoxic Training systems and has not added artificially-induced hypoxic conditions to the 2007 List. The Committees found that the method was performance enhancing, determined that the methodology was contrary to the spirit of sport, raised some concerns but was inconclusive about the method's threat to athlete health. A substance or method may, but is not required to, be added to the Prohibited List if it meets two of these three criteria. There's been a lot of discussion regarding the preliminaries for this decision and the subsequent misinformation surrounding the committee's determination that Hypoxic Training is contrary to the spirit of sport. Here are a few excellent resources for those whose mind is not yet made up: Interview with Dr John Hellemans of the New Zealand Academy of Sport South, links to WADA submissions from various authorities, and an excellent letter from Doriane L. Coleman , Professor of Law at Duke Law School, and an affiliate of Duke Law School's Center for Sports Law and Policy.