You're probably familiar with these games already - I'm sure SOMEONE must have mentioned them here at some point in the last few years (I may have done it myself, I don't recall). But if not...The Last Stand
and The Last Stand 2
. Entertaining (if often frustrating) zombie siege flash games. Hold back the zombie horde at your barricade all night, then spend the daylight hours doing repairs and searching for weapons, supplies and survivors. Fun lil' timewasters both, though #2 is obviously more in-depth and complicated.