Assuming your server runs on Bukkit, there are a few mods that can increase survival difficulty.
Allows you to change the damage done by mobs.
Ever felt like something was missing from Bukkit? A sense of immersion, of depth? Ever felt like people shouldn't be able to survive for weeks without a single piece of food? Gastronomic provides a reason to kill those pigs, catch that fish, and cook that bread. So gather up your wheat, mushrooms and pork and keep reading.
Want to add some additional difficulty to survival? This plugin will allow zombies to break blocks of your choice to get to the targetted player. You choose the blocks you want zombies to be able to break and the time in seconds it takes for the zombie to break that block type. WorldGuard can be used to protect regions from zombieattacks. Permissions lets you set what player/group is under attack.
This plugin offers the option to have some (or even all) wolves that spawn be already angry.
As per the title, this plugin makes it so giants will spawn naturally along with other mobs.
Giants are set to be more rare, and by default will only spawn 5% of the time compared to other mobs. This option, along with other options, can be changed within the configuration file. Giants additionally will only spawn in the dark / at night.
TimeShift gives you control over how time progresses on your server. It allows you to set both the current behavior and the startup behavior for any world to progress normally, loop day, or loop night. It also offers some options for sunrise and sunset.