When looking at charities, things aren't always what they seem. Be especially cautious of those administrative cost ratios. When a charity mails you a donation request with some information about their problem of interest, the costs will be attributed to public education, not an administrative cost of fundraising.
Dumping a lot of donated stuff at an airport is cheap and it allows a charity to brag about low administrative costs. Americares is notorious for that - they collect manufacturer over-runs, close to expiration pharmaceuticals, etc. and carry them overseas. Their administrative costs pale against the "value" of what they provide and their corporate "benefactor" get a big tax write-off instead of a disposal cost.
One reason for microcredit's high administrative costs is that their usual business model requires a network that reaches into the smallest rural communities. That's expensive. Grameen is just the most famous of many microcredit organizations. I've been following microcredit for a long time and wrote a 20+ page term paper last year on the social background of microcredit in Bangladesh.
Microcredit organizations know they can score big international support by marketing their focus on women borrowers (plays well with social liberals) or that they "help people help themselves" (plays well with fiscal conservatives). The fact is that they prefer women borrowers because they're easily threatened in a patriarchal society when payments are due. And many of the borrowers pay off a loan from one organization with a loan from another so, in addition to all their other struggles, they also find themselves in debt.
Donate whatever money you can, but also look around for something you can give some of your time to. I help out with an affordable housing organization and my wife helps run a flower sale for a local shelter and she & my daughter help raise money for Heifer International. We donate money to them and others, but we're not quite in Bill Gates' financial strata and our time does them more good.
By the way, there's a real interesting book about international charities, called The Road to H*ll. With an e instead of a *, but I had to beat the dreaded Terraforum filter.