Intramuscular testosterone undecanoate is available in the United States , Canada , Europe , and elsewhere in the world.    It is approved in over 100 countries worldwide.   Oral testosterone undecanoate is available in Canada, Europe, Mexico, Asia, and elsewhere but not in the United States.   Intramuscular testosterone undecanoate is marketed most commonly as Nebido in Canada and Europe and as Aveed in the United States while oral testosterone undecanoate is marketed most commonly as Andriol.   
For the off-season athlete there is no anabolic steroid more important or beneficial than testosterone. High levels of testosterone will promote significant increases in lean muscle mass and strength. This is assuming that the individual is consuming adequate calories. Compounds like Testosterone Propionate are not magical, you will still need to feed your body enough calories. During an off-season period of growth, this means total caloric intake will need to be slightly above maintenance. This will, unfortunately, promote body fat gain. However, the key to a successful off-season is gaining lean tissue while minimizing body fat gain to the fullest extent possible. By supplementing with Testosterone Propionate you will be able to achieve this more efficiently. High testosterone levels will promote a stronger metabolic rate. This is not a license to eat like there’s no end in sight, but you should be able to make better use of your calories.
Testosterone esters were synthesized for the first time in 1936, and were found to have greatly improved potency relative to testosterone.  Among the esters synthesized, testosterone propionate was the most potent, and for this reason, was selected for further development, subsequently being marketed.  Testosterone propionate was introduced in 1937 by Schering AG in Germany under the brand name Testoviron.  It was the first ester of testosterone to be introduced,  and was the major form of testosterone used medically before 1960.  In the 1950s, longer-acting testosterone esters like testosterone enanthate and testosterone cypionate were introduced and superseded testosterone propionate.  Although rarely used nowadays due to its short duration,  testosterone propionate remains medically available.