A good "free" workout actually involves gaming and/or TV. It's easier with TV, but you can modify for gaming.
Every commercial break do 15 crunches and 10 pushups, after a while you can bump it up to 10 crunches and 5 pushups per commercial (assuming you know your shows commercial routine). For gaming I do 50 crunches and 30 pushups every hour. It's a bit harder with RPGs, but right now with the 30 second + wait between matches in BF3 I'm doing 20/15. I'm no expert and I'm borderline getting a civilian job because I struggle with the military fitness test, but the above examples at least maintain me enough to keep me above the border ^.^