Probably you have. If you work for a branch of government or a major corporation, it's hard to miss the appeals.
I'm a United Way donor because I don't have the time or energy to figure out which organizations I should donate my money to and figure even if I did figure it out, I probably wouldn't give to a worthy one because of some unfortunate bias on my part.
If you don't have a handy way to donate and want to, you can go to this link.
If you would feel better about donating if a famous celebrity suggested it, click here to see Céline Dion.
Some people recommend giving directly to the United Way organization of your choice because that way your favoured organization gets 100% of your donation. That makes sense if you really are going to give the money to the organization, but not if it's just a way of procrastinating and not donating anything.
As for the cut that the United Way takes to fund itself, many of the organizations the United Way funds would fold without the United Way money. The ones that didn't fold would have to use their own resources to raise their money and they would not be as efficient.
[Note: The United Way is having a 75+75=31 campaign that they asked me to blog about, but that sort of thing makes me squeamish. If campaigns with bad arithmetic slogans or Céline Dion messages have a similar effect on you, ignore those pages and look at the agencies they support.]