Voluntary Charity in Islam

[truthseeker63]

I know there is Zakat in Islam but is there a such thing as Voluntary Charity in Islam which I think is called Sadaqah can one give as much as he wants what about Zakat what if a Rich/Wealthy person pays 2.5% but he wants to pay more of his wealth can he pay more Zakat or does he has to use the Sadaqah also what if there was a large famine in the land and Zakat could not raise enough wealth to help people at just 2.5% ?

[Ramadhan]

Abu-Hurairah quotes the Prophet (Pbuh) as saying: "There will be in the distant future famine. He who lives through it should equal nothing with feeding the hungry" (Related by Al-Bukhari in Al-Adab Al-Mufrad). When people are starving and are in need of the smallest amount of food to survive and when there is nothing available to relieve their hunger, coming forward with supplies to feed such victims of famine is the greatest act of charity. It is the way to ensure the best reward from Allah.
http://www.islamicvoice.com/january.97/hadi.htm

Also, the 2.5% rate of zakah is the minimum:

It should also be remembered that the rate of 2.5% is only a minimum. In times of emergency or arising needs there is no rate limit; the more one gives, the better it is for all concerned. The distribution of Zakah serves all purposes for which numerous funding raising campaigns are launched. The Zakah fund substitutes for all the other funds. It is authentically reported that there were times in the history of the Islamic administration when all people of the vast Islamic empire had enough to satisfy their needs, and the rulers had to deposit the Zakah collection in the Public Treasury. This shows that when the Zakah law is enacted properly it minimizes the needs of the citizens and enriches the Public Treasury to such an extent that there may be no needy or poor and that enormous amounts of surplus are available.
http://www.islamicbulletin.com/newsletters/issue_6/zakat.aspx

Zakah is paid on the net balance after a Muslim has spent on basic necessities, family expenses, due credits, donations and taxes. Every Muslim male or female who at the end of the Hijri year is in possession of the equivalent of 85 grams of gold or more in cash or articles of trade, must pay his or her zakah at the minimum rate of 2.5%.
mosquepraiseallah.com/wp-content/uploads/Zakah.pdf