Effective home fitness equipment only cost me AU$37 on ebay. Chin up bar with varying grips, plus 'ab straps'. This fits easily onto most standard door frames.
Chin ups are the "squat" for the upper body. You get huge bang for your buck doing chin ups, work several muscle groups at the same time, and you don't need to go to the gym all the time or at all to work on developing super strength like this. In fact you can have chin up competitions at home with your family or friends so it also makes for a good party trick device
The great thing about having a chin up bar at home is you can practise at any time, and even if you haven't done a chin up ever before and can't when you first start, if you persist you will likely manage one soon enough, then another, then another. It is a very satisfying experience being able to do chin ups, then you can go to the gym and show off your skills
The ab straps can help you to develop your core and ab strength, which ties in beautifully with chin ups. Ab straps are a great addition to the chin up bar (and cost $11 out of the $37 I had spent on my combined contraption).
For tips on getting started with chin ups, or how to further develop and improve if you are a bit ahead beginner status, or even just for an entertaining read on the history, beauty and significance of chin ups, see here: http://www.charlespoliquin.com/ArticlesMultimedia/Articles/Article/567/Chin-ups_The_King_of_Upper_Body_Exercises.aspx