One blog I subscribe to is Mike Heiser's The Naked Bible. It is challenging, deep, and thought-provoking. He posted a two-part article by Andreas Kostenberger and David Croteau on tithing. You can access them here. The first article is a thorough examination of tithing in the Old and New Testaments. The second article is an equally thorough examination of systematic issues and principles related to tithing, as well as the establishment of a biblical model for giving.
In short, Kostenberger and Croteau conclude:
No system of tithing was present in the pre-Mosaic period, nor was there a command to tithe. Tithing was voluntary before Moses arrived.
The annual giving of the Israelites under Moses far surpassed ten percent. The only proper recipients of the tithe were the Levitical priests, who have been replaced by all Christians (and not just pastors).
Tithing in the Historical and Prophetic books of the Old Testament have contexts so specific that they cannot be used in support of tithing.
None of the three New Testament passages that mention tithing can be appropriately used to continue tithing under the new covenant. In none of these is tithing the main subject or the ultimate point of reference.
The theological argument in favor of tithing because Jesus came to "fulfill, not abolish" the Law does not properly account for its meaning or consequences. Jesus' fulfillment of the Law neither is neither an argument for or against tithing. You have to account for what the tithe was and how it functioned in the Mosaic law; namely, tithing in the Mosaic law was always tied to the sacrificial system. The sacrificial system has been fulfilled by Jesus on the cross.
The theological argument that tithing is a part of the "moral law" and thus should continue is flawed. Apart from the fact that separating the Mosaic law into three compartments (civil, ceremonial, and moral) is problematic, tithing is never connected to the "moral law" in the Old Testament.
The argument that tithing should continue because of traditionalism ("we've always done it that way" or "if we tell people they don't have to give 10%, offerings will decrease") is also wrong. Scripture, not tradition, is to be our guide, and we should not impose requirements upon believer's unless Scripture commands us to.
Neither is the argument that tithing is pragmatic correct. Just because it is easy to tell people to divide their salary by ten doesn't make it the biblical thing to do. Doctrine can never take second place to pragmatism.
There are other problems with obligatory tithing: (1) Nowhere in the New Testament are Christians commanded to tithe. (2) There were multiple tithes in the Old Testament. To call for the cessation of two of them while maintaining one is logically unsustainable. (3) None of the New Testament's passages on giving ever mentions a percentage.
The following is a summary of the New Testament's teaching on giving:
Systematic - Give on a regular basis (weekly, monthly, etc; 1 Cor. 16:1)
Proportional - Give as you have prospered, according to your ability (1 Cor. 16:2; 2 Cor. 8:2-3)
Sacrificial - Give generously, but not to the point of personal affliction (2 Cor. 8:2-3; Phil. 4:17-18)
Intentional - Give deliberately in order to meet a genuine need, not out of guilt merely to soothe a pressing request (2 Cor. 8:4; Phil. 4:16)
Motivational - Give out of love for others (2 Cor. 8:9), so everyone's needs can be met (1 Cor. 9:14-15; 2 Cor. 8:12-14; Gal. 6:6), and so you can receive more from God (in order to bless others more; 2 Cor. 9:6)
Cheerful - God loves a cheerful giver (2 Cor. 9:7)
Voluntary - Giving should be done out of one's free volition (2 Cor. 8:2-3, 8; 9:7; Phil. 4:18)
Giving comes down to our relationship with the Father. We should be willing to give whatever the Lord asks, whether it is ten percent, twenty-five percent or one hundred percent. We should not use our freedom from an obligatory ten percent tithe to hoard the money God has given us. If we do that, then our relationship with our Father is harmed.