Free Money Finance argues that the old covenant tithe command applies to Christians. I was going to leave a comment, but I decided I might as well make it a post. The topic has come up here before. Wink tried to get a discussion going on tithing last summer. Also, much of what I'm going to say has a background much more carefully drawn out in Christians and the Sabbath and More Sabbath Stuff.
One of the arguments in the post is that Abraham gave a tithe long before the law of Moses. From this it is concluded that the tithe principle must be eternal and thus not just a particular command to the people of Israel in the Mosaic law. There are a number of things that someone could say about Abraham's tithe, but one thing you can't say is that he was following any command from God that he give 10% of his income to God. He wasn't giving it to God, for one, and we have no information about any command he was following, never mind a command as to the exact amount. A gift of 10% to a benefactor was probably just a common ancient near eastern practice that the Torah adopts because the symbolism of giving firstfruits to God as representative of everything you have belonging to God needed some amount. For the particular command to the particular people of Israel to give some amount as firstfruits, God seems to have chosen the amount that for whatever reason had already been standard in that part of the world at that time, as evidenced by Abraham's gift to Melchizedek. The more important principle is that everything we have is God's, with the firstfruits we give to him standing for that.
10% isn't some magical amount. The Torah uses different percentages to determine the firstfruits amount for other things. With the tithe of time, it's 1/7 of all the days in the week rather than 1/10. With the tithe of the firstborn, it's one out of however many children there end up being, which is 100% when there's only one but less than 10% if there are more than ten).
In the comments, some people were accusing anyone who took an alternative view of simply trying to justify giving a lower amount, even degenerating into speculation about who might be giving a higher amount than someone else. It's irrelevant why someone offers an argument. If it's a good argument, it's a good argument. That's true regardless of why someone likes the argument and tries to use it to persuade someone. People who aren't very generous might latch on to this sort of argument to justify stinginess, but assuming that's what's going on is thoroughly immoral. Why should a Christian assume the worst of a brother or sister's motivations?
For the record, I think some, perhaps most, people have a moral obligation to return much more than 10% of their income. None of it belongs to them anyway. I just think it's against the new covenant teaching to assume that some specific amount that is only given as a command within the already-fulfilled old covenant will apply outside the tightly-defined group that the old covenant applied to. (Jesus' statement to the Pharisees who were still under the old covenant does not count.)