The Last Stand 2
You play a man trying to get to the coast for evacuation- you spend your days searching for other survivors to help you, more weapons, supplies to make the journey to other cities, and your nights defending yourself from the attacking horde.
The graphics are cool, the atmosphere is great, and this game is full of a lot of nice touches, which make it fun to play. It's also pretty tense, but in a good way. When you inevitably die, the first thing you'll want to do is try again.
If you like the Last Stand 2, another online zombie game with a similar style is Cursed Village, which you might also enjoy.
You can play Last Stand 2 here.
Want to see more cool online zombie games? Read my list of the very best ones.