R - cumulative sum by condition

Using data.table:

require(data.table)
DT <- data.table(DF)
DT[, Cum.Sum := cumsum(Sum), by=list(Year, ID)]

    Year  ID Sum Cum.Sum
 1: 2009 999 100     100
 2: 2009 123  85      85
 3: 2009 666 100     100
 4: 2009 999 100     200
 5: 2009 123  90     175
 6: 2009 666  85     185
 7: 2010 999 100     100
 8: 2010 123 100     100
 9: 2010 666  95      95
10: 2010 999  75     175
11: 2010 123 100     200
12: 2010 666  85     180

Another way

1) use ddply to sum a variable by group (similar to SQL group by)

X <- ddply ( dataset, .(Year,ID), sum)

2) merge the result with dataset

Y <- merge( dataset, X, by=('Year','ID')