@EratiK: I saw that book but I'm quite dubious. They say without machines but nearly every pose involves using furniture or props in one way or another. I mean, go find two perfectly similar chairs that are high enough and stable to use for dips, for example. They're not in my appartment. I think the intention is sound, but in practice, unless you wanna stick to Indian pushups and Indian squats... you need something to exercise with.
You think it's expensive? Hmm. There's
one for just 170 € but I suspect it'd be less stable.
The idea with the machine I've linked to is that it uses multiple muscle groups (well,
chinups do, maybe dips not so much).
Well that's a pull up bar which you can also do chin ups, dips (the handles), and abdominals. Actually there is a more compact version with the pull up bar on the same side as the dip handles, so you can put one side to the wall.
I guess the companion to that machine would be a squat rack for all the lower body workout with the almighty squats.
@jettyke: I have a bad upper back since spending so much time at my computer in my teens. When I went to the gym, doing exercises like
the pulldown had a positive effect in that I was holding my back better I guess due to it being "toned". But frankly I couldn't bother going to the gym, I'd rather do core exercises like above, and use Yoga for everything else.
You asked for direct experience. There is one extremely simple Yoga exercise I do whenever I have a minute, and I can grab something at waist hight. It's called
wall plank (scroll down).
I can't say that it will strenghten your back, but it will certainly undo the pressure and stress caused by bending forward at the computer. It will also help to open up your shoulders. This is actually a preparatory exercise for the full forward bend. I can tell you from experience just doing that wall plank 2-3 times a day for just a minute, you'll be amazed you can forward bend and almost lie your hands flat on the floor.
Try to do this with a chair or a table, and put your hands flat, it's a lot easier than the wall. You need something at waist height. The key is to tense your abdominals and upper front thigh so that it pulls gently your waist down, flattening your back, and also pulls your waist backwards to elongate the spine. Also rotate your biceps so that they turn inwards, so that you feel your scapula flattening on the back. Breathe slowly and fully.