The sharia also contains tax system, Zakaat is at lowest at those 2,5% and they are ment to be paid if you are avaiable to do it and don't have loan. If you wish to give more zakaat, ur welcome to give more.
I believe tax is good for the country. Look at Denmark, we pay 35-60% tax (depending on how much we earn), and the roads and schools are all prosporous.
[21:73] "We made them (Abraham and his sons) imams who guided in accordance with our commandments, and we taught them how to work righteousness, and how to observe the Contact Prayers (Salat) and the obligatory charity (Zakat). To us, they were devoted worshipers."
[17:26-29] You shall give the due alms to the relatives, the needy, the poor, and the travelling alien, but do not be excessive, extravagant.
The extravagant are brethren of the devils, and the devil is unappreciative of his Lord. Even if you have to turn away from them, as you pursue the mercy of your Lord, you shall treat them in the nicest manner. You shall not keep your hand stingily tied to your neck, nor shall you foolishly open it up, lest you end up blamed and sorry.
[6:141] "Eat from their fruits, and give the due alms on the day of harvest"
[2:215] "They ask you about giving: say, "The charity you give shall go to the parents, the relatives, the orphans, the poor, and the traveling alien." Any good you do, God is fully aware thereof."
[9:60] Charities(Sadaqaat) shall go to the poor, the needy, the workers who collect them, the new converts, to free the slaves, to those burdened by sudden expenses, in the cause of Allah, and to the traveling alien. Such is GOD's commandment. GOD is Omniscient, Most Wise.
[2:3] who believe in the unseen, observe the Contact Prayers (Salat), and from our provisions to them, they give to charity
___
This explains about the 2,5% rate
http://www.easyislam.com/zakat.asp