The Last Stand series, by Con Artist Games, are some of my all-time favorite games. The Last Stand is a very simple game where all you have to do is survive. You're a guy taking on hoards of zombies, trying to prevent them from taking down your trusty barricade you've worked so hard on building. You play over the course of 20 days and nights (fighting the zombies each night). During the day you decide how to spend your time. You can either look for weapons (there are 11 different ones - see if you can get the big boy), look for other survivors to help you defend your barricade, or repair your barricade from any damage it may have received. Can you survive the 20 days of undead?
Next in the series is The Last Stand 2. This installment is almost the same as the first but with a few welcome additions. This time you must make it through Union City over the course of 40 days and nights to reach a helicopter that will take you off the infested mainland. You can decide which surrounding buildings to search through to find supplies (which are needed to move through Union City), weapons (now including traps), and survivors (that you can equip with whatever weapon you choose), and my personal favorite - you have a cool new beard.
The Last Stand: Union City is still being developed (currently Alpha 0.96). This third installment should be massive compared to the first two. There are more buildings to enter, more weapons, you can now accessorize what you wear (who wouldn't want to look fabulous while killing zombies?), items can be used to increase your stats, different zombies types are roaming around, quests are now available, the list goes on. I cannot wait for the final release.