Donor Streak Formula

/*Already donated this year*/ 
if(year(npo02__LastCloseDate__c)=year(today()),1+ 
if(contains(Years_Donated__c,text(year(today())-1)),1+ 
if(contains(Years_Donated__c,text(year(today())-2)),1+ 
if(contains(Years_Donated__c,text(year(today())-3)),1+ 
if(contains(Years_Donated__c,text(year(today())-4)),1+ 
if(contains(Years_Donated__c,text(year(today())-5)),1+ 
if(contains(Years_Donated__c,text(year(today())-6)),1+ 
if(contains(Years_Donated__c,text(year(today())-7)),1+ 
if(contains(Years_Donated__c,text(year(today())-8)),1+ 
if(contains(Years_Donated__c,text(year(today())-9)),1+ 
if(contains(Years_Donated__c,text(year(today())-10)),1+ 
if(contains(Years_Donated__c,text(year(today())-11)),1+ 
if(contains(Years_Donated__c,text(year(today())-12)),1+ 
if(contains(Years_Donated__c,text(year(today())-13)),1,0),0),0),0),0),0),0),0),0),0),0),0),0), 

/*donated last year but not yet this year*/ 
if(year(npo02__LastCloseDate__c)=year(today())-1,1+ 
if(contains(Years_Donated__c,text(year(today())-2)),1+ 
if(contains(Years_Donated__c,text(year(today())-3)),1+ 
if(contains(Years_Donated__c,text(year(today())-4)),1+ 
if(contains(Years_Donated__c,text(year(today())-5)),1+ 
if(contains(Years_Donated__c,text(year(today())-6)),1+ 
if(contains(Years_Donated__c,text(year(today())-7)),1+ 
if(contains(Years_Donated__c,text(year(today())-8)),1+ 
if(contains(Years_Donated__c,text(year(today())-9)),1+ 
if(contains(Years_Donated__c,text(year(today())-10)),1+ 
if(contains(Years_Donated__c,text(year(today())-11)),1+ 
if(contains(Years_Donated__c,text(year(today())-12)),1+ 
if(contains(Years_Donated__c,text(year(today())-13)),1,0),0),0),0),0),0),0),0),0),0),0),0),0))

Tweaked the code to get 20 years of consecutive giving (or 21 if they gave this year). If someone else uses this, just make sure to replace "Years_Donoated__c" with the name of your custom field that contains the list of years that they donated.

if(contains(Years_Donated__c,text(year(today()))) || contains(Years_Donated__c,text(year(today())-1)),
 if(contains(Years_Donated__c,text(year(today()))), 1, 0) +
  if(contains(Years_Donated__c,text(year(today())-1)),1+ 
   if(contains(Years_Donated__c,text(year(today())-2)),1+ 
    if(contains(Years_Donated__c,text(year(today())-3)),1+ 
     if(contains(Years_Donated__c,text(year(today())-4)),1+ 
      if(contains(Years_Donated__c,text(year(today())-5)),1+ 
       if(contains(Years_Donated__c,text(year(today())-6)),1+ 
        if(contains(Years_Donated__c,text(year(today())-7)),1+ 
         if(contains(Years_Donated__c,text(year(today())-8)),1+ 
          if(contains(Years_Donated__c,text(year(today())-9)),1+ 
           if(contains(Years_Donated__c,text(year(today())-10)),1+ 
            if(contains(Years_Donated__c,text(year(today())-11)),1+ 
             if(contains(Years_Donated__c,text(year(today())-12)),1+ 
              if(contains(Years_Donated__c,text(year(today())-13)),1+ 
               if(contains(Years_Donated__c,text(year(today())-14)),1+ 
                if(contains(Years_Donated__c,text(year(today())-15)),1+ 
                 if(contains(Years_Donated__c,text(year(today())-16)),1+ 
                  if(contains(Years_Donated__c,text(year(today())-17)),1+ 
                   if(contains(Years_Donated__c,text(year(today())-18)),1+ 
                    if(contains(Years_Donated__c,text(year(today())-19)),1+ 
                     if(contains(Years_Donated__c,text(year(today())-20)),1,0)
                    ,0)
                   ,0)
                  ,0)
                 ,0)
                ,0)
               ,0)
              ,0)
             ,0)
            ,0)
           ,0)
          ,0)
         ,0)
        ,0)
       ,0)
      ,0)
     ,0)
    ,0)
   ,0)
  ,0)
,0)