At DonorsChoose, their motto is Teacher's Ask, You Choose. Basically, teachers register at their site and request items for the classroom. People like you or I can then go to their site and pick which projects to fund. You can choose based upon location, grade level, or subject, along with some other criteria. You don't have to fund the whole project. They pool the donations from everyone such that lots of little donations can be made to fund a really expensive project (or even a less expensive one, you just don't need as many little donations).
The types of projects they fund are incredibly varied. You can fund one time things like an author coming to a classroom, or something that will get reused year to year like a new projector. Personally, I like the projects where the items are reusable beyond just the current students.
One thing I like about the site is the feedback you get from the teachers, both while the project is being funded and afterwards. For one particular project I helped fund, I was actually sent letters from each student. They were great to read. With the photos the teachers post, you can actually see what you helped purchase being used.
The great thing about the place is many times foundations will match donations. So if you donate $25, thanks to the foundation, the project will get $50. They let you and I decide which project they will help fund. Sometimes, the foundations will just outright fund all the projects in a particular state, like what the Claire Giannini Fund did back in 2010. They'll even email you periodically $25 "gift" cards which you can use to fund whatever you'd like. The favorite thing I ran across with them is the time they sent me four or five $25 gift cards. Not for me to use myself, but to give to other people, to introduce them to the site and let them pick what they wanted to fund. Obviously, the hope was they would come back and donate their own money, too, but that wasn't a requirement.
Today, when I went to look at the projects to fund, the default criteria was Highest Poverty and Closest to Finish Line, with the hope that these projects would be fully funded next. At the top of the list is Mic Check 1, 2 where Ms. Maez of Denver, Colorado is looking to pickup a portable sound system that would cost over $500 for fulfillment. Eight people have donated so far and they're $49 away from completion. On the more expensive side, are projects like the Teaching Young Minds Through Technology project. Here, Mrs. Cox is looking to pickup two desktop computers and one laptop for her classroom to the tune of $3,615.47. So far nine people have donated a total of $552 so they only need to collect $3,063.47 more. On average each person only donated about $60. I've seen averages even lower. So if you find a project you are interested in helping out, even $5 would help.
Disclosure: No compensation was received for this post. See my disclosure policy for more information.