These resources were excerpted from this post on NPR.org.
Here are some charities with an on-the-ground presence and accepting donations:
My Brother's Workshop is a St. Thomas-based charity that has been running a daily lunch program on the island.
Americares delivers medicine and aid to local health providers and is readying emergency kits for future crises.
GlobalGiving is supplying emergency supplies across the Caribbean in addition to longer-term recovery help.
All Hands Volunteers is a volunteer-driven disaster relief organization awaiting "the green light" from first responders to get back to St. Thomas to help.
Catholic Relief Services is working with local governments across the Caribbean to provide relief including shelter, water and kitchen kits to hurricane victims.
The Florida-based National Puerto Rican Leadership Council Education Fund is helping coordinate relief efforts. President Carlos Guzman tells NPR they are accepting diapers, formula and water. But he adds, "money is better because all the airports are shut down," and the group is trying to get goods over the island via private jets.
The Mexican Red Cross or Cruz Roja Mexicana has teams assisting in search and rescue operations. It has also put together an Amazon wish list of needed items, including tents, baby supplies and batteries.
Oxfam is asking for funds as it develops a response plan in coordination with the Mexican government and other aid organizations.
Save the Children is raising money for a children's relief fund.
How to make your money go further
"As when giving with any charity, try and do your due diligence about the organization you are giving to. Do friends vouch for it? Has it been endorsed by other trustworthy organizations? And watch out for scams," reports NPR's Carrie Kahn from Mexico City.