Modules

The Linux kernel is monolithic (not micro kernel), but supports LKMS - Loadable Kernel Modules:

Device drivers
Load only the driver modules you need.
Optional code for kernel functions	
Select only the netfilter modules you need.
Kernel code needed for certain user-space functions.

Also good for

Honeypot monitoring tools
Patching/disabling buggy kernel calls without upgrade
Rootkits / circumventing security