You are your own Gym!
For the practitioner of calisthenics, his or her body becomes their own gymnasium. Most body weight exercises require no equipment, except for a place to pull yourself up. You can purchase of a pull up bar for less than £20, but all you need is a place to hang from; a door frame, below a stairwell, or even the branch of a tree in your garden or local park!
Whereas other strength training systems use weights, cables or machines to produce resistance, the vast majority of calisthenics exercises exploit a free form of resistance, namely gravity. This means that you can train anywhere you happen to be; a hotel room, on holiday, in a different city, outside, at work - anywhere. In fact training in this way, away from the restrictions of a busy gym, can offer a much better workout. I am not saying that gyms are useless (on the contrary, they are great places to train and to hook up with others who think likewise) but training knowledge is power and this is what body weight training will provide you with.