I like a lot to work with the Aforge Library that has two good FlodFill implementations, but both are 4 directional only.
If you need other implementations there exist two code project articles Flood Fill Algorithms in C# and GDI+ and Queue-Linear Flood Fill: A Fast Flood Fill Algorithm.
And if you like to dig in the code and learn, there is the Paint .Net project that has an excelent flood fill implementation.
