We used to have a weight machine here at home, but we sold it before me moved cross country. Talked about getting a new one, but instead we bought one of those pilates balls and some free weights.
I do balance and core exercises on it as well as crunches and some back exercises. My husband does pushups on the ball and uses it as a weight bench with the free weights. Apparently a lot of people are going to the ball instead of a weight bench because the ball makes you keep your abs and obliques tight while you're lifting with your chest and arm muscles.
I bought the ball at a sports store nearby for about $20. Worth a try.
Oh.. and I also sometimes use it as a chair in my office when I'm typing a lot. Balancing on the ball requires you to work your core muscles while you're just sitting at your desk. I'm still a long way away from abs of steel, but I'm getting there.