A small snack, a pat on the back, and a small sticker.
PLUS:
A big warm fuzzy feeling. I’ve seen what donated blood can do - I get happy, helping that along.
I can’t answer as a donor any longer (arthritis meds disqualify me) but SWMBO is an O-negative donor and she’s well over 10 gallons so far. Got a cup for every gallon and a bunch of goodies for that 10th one.
Pizza? Man, I’m donating in the wrong city!
I have received t-shirts, cups, gallon pins and best of all I get karma points. After I reached 6 gallons, I received an invitation to be a part of a United Blood Services commercial and Promotional video. I was asked to tell my story of why I donate and what I have done since being a recipient of someone else’s blood. I lived in NM at the time of the filming but I have actually seen the video being played in other states. Once I was meeting a friend of mine who was working at a health fair in Portland, Oregon and a little girl recognized me from the video which was playing at a UBS booth. My friends son, maybe 6 years old, thought it was super cool when he heard the little girl tell her mother that I was the guy she just saw on TV. 
Nothing. I went once when they were doing a blood drive, and they had crackers, but I had my sandwich and brownie waiting in the car as usual.
Back in Ireland I feasted on a faceful of free cakes afterwards and could have had a full on lunch - you got a pin after so many donations.
The best reward was a doctor rabbiting on about the effects a transfusion had had on his very sick patient.
I donated successfully for the first time at Dragon*Con this year, so I got a t-shirt.
Like the guy says, “Save a shirt, get a life!”
I get time off work to donate.
When the blood bank came around to our high school they’d give us a cookie or doughnut. But the real reason people donated was to get out of a class!
In college I started selling my plasma which ended my altruistic streak, as far as bodily fluids were concerned. :-/
The one and only time I was able to donate I got told I could get a T-shirt or a duffel bag. I took the bag. I’ve used it a number of times.
I have more Oklahoma Blood Institute t-shirts than I can count. They also give points for various catalog merchandise. I usually use mine to buy a yearly PSA test. Plus, since our blood drive is always at work, my employer gives us an additional two hours off on the day of the donation. That doesn’t even count the time involved in giving.
I donate three times a year which is how often they come to my place of work. Usually it’s just snacks but one I got a t-shirt, once a certificate for a car wash and once a burger and fries coupon (probably at the same place as iamthewalrus.) I also get points (double points when I do the double red cell thing) that are redeemable for movie tickets but I’ve never used them.