There are allot of mods (and a lot of methods) that come to mind. I'll try to list some of the easy to build and fun to see.
From the mod : ICBM
The gun and laser turret.
These are specifically designed to keep your base safe. You can program them to attack players or to only attack certain players and all mobs.
From the mod : Industrial craft
The Tesla coil.
The force field (MFFS)
From the mod : Thaumcraft
Iron Golem Guardians can be placed anywhere. When placed they will
guard an area around their original location from hostile mobs, though
not creepers. When crafted with the intelligence core the golems can
be told to target specific mobs, this includes creepers. However when
an iron golem attacks a creeper it will attempt to avoid the blast.
Iron Golems also repair their damage twice as fast as the other
golems. Iron Golem Guardians may also be given upgrades as shown below
in the Golem upgrades section.
Please note that these golems are considered Tier 3 and require a
great deal more research than the other golems
There are a lot of methods to get rid of mobs. I'm pretty sure i missed some of the fun ones but these are quite effective on their own.