There is this very, very old puzzle/observation in economics about the lawyer who spends an hour volunteering at the soup kitchen, instead of working an extra hour and donating the money to hire someone to work for five hours at the soup kitchen.

There's this thing called "Ricardo's Law of Comparative Advantage". There's this idea called "professional specialization". There's this notion of "economies of scale". There's this concept of "gains from trade". The whole reason why we have money is to realize the tremendous gains possible from each of us doing what we do best.

This is what grownups do. This is what you do when you want something to actually get done. You use money to employ full-time specialists. -- Eliezer Yudkowsky

Similarly, when you give stuff, it's probably not the right stuff or in the right place. There are lots of problems with trying to use donated stuff for aid work, including limited transportation, dumping, storage space, and inconsistency. Claire Durham of the Red Cross describes the problems very well.

To maximize the good you do (as opposed to getting personal fulfillment through volunteering or getting rid of unwanted stuff), give whatever charity organization you're working with what they can do the most good with: money.

[1] This is mostly if you're serving soup or helping build houses. If you're providing professional services that the charity would otherwise have to purchase (a lawyer volunteering to do legal work) then you're cutting out some of the overhead of hiring people.