Ah, the tithing debate. It's astounding to me how people can read the same Bible, the inspired Word of God, and come up with 400 different "interpretations" of what it says.
First, I believe in tithing. I tithe 10% of my gross income (read: first fruits). I do so because it is a direct commandment from God. Since I love God with all my heart, I want to honor his commandments. That's the attitude with which I tithe. I never consider tithing as an obligation. It's all in your attitude, and God knows your heart.
That said, I believe that tithing is just the beginning. Many times, God will speak to me or my wife to give to a person or a ministry. This is what is called an offering. These amounts are between me and God. But my tithe is non-negotiable.
Why do I believe this? Because it works! God's word is true. In Malachi, God asks us to challenge him. He tells us to bring all the tithes AND offerings into the storehouse and see if he won't open the windows of heaven and pour a blessing out that you won't have room enough to receive. But is it simply for personal gain? No. It's so that you will be able to be an even bigger blessing in the future.
I have seen over the years in my own life, that no matter what financial setbacks we've experienced, God has always provided a way of escape. I believe this is because it also states in Malachi that when you tithe, God will rebuke the devourer for your sake. Why on earth would God do away with tithing when it is clearly an avenue for him to bless us?
A lot of people say they can't afford to tithe. It's been my experience that you can't afford not to tithe.