For charitable donations in Canada:
In 2007, the first $200 you donate is eligible for a federal tax credit of 15.25% of the donation amount. After the first $200, the federal tax credit increases to 29% of the amount over $200. Generally, you can claim all or part of this amount up to a limit of 75% of your net income. For gifts of certified cultural property or ecologically sensitive land, you may be able to claim up to 100% of your net income.
For donations to a political party in Canada:
Income tax credits
When Parliament changed the Canada Elections Act, it also changed the Income Tax Act to allow higher income tax credits for political contributions by an individual:
* for contributions up to $400, a credit of 75 percent (for example, a $300 credit for a contribution of $400)
* for contributions from $401 to $750, a credit of $300 plus 50 percent of the amount over $400 (for example, a $475 credit for a contribution of $750)
* for contributions over $750, the lesser of $650 or $475 plus 33⅓ percent of the amount over $750 (for example, a $650 credit for a contribution of $1,275)
EDIT: As someone points out in the comments, your tax credit is hardly the end of it - it's hard to say, really, how much a $100 donation to a political party costs the taxpayer. $75 upon donating, plus if you're near an election, you can add 60% of any the donation spent in that election... plus 60% of any of that refund spent in the next election. It's safe to say, though, that every time you donate to a political party, especially during an election, you're probably going to cost the taxpayers as much as you're willing to pay - and maybe more.