In-kernel web server

An in-kernel web server is an unlimited HTTP server that runs in kernel space or equivalent. It is also known as "accelerator".

Benefits

Performance: the path taken by data from a source device to a destination device. Proper asynchronous zero-copy interfaces would make this available from user-space.
Scalability: with respect to number of simultaneous clients. Event notification of comparable scalability seems unlikely in user-space.

Drawbacks

Security: Kernel processes run with unlimited privileges.
Portability. Every kernel needs a specific implementation route.
Reliability. Failure in the webserver may crash the OS.

Implementations

illumos/Solaris: NCAkmod aka Network Cache and Accelerator kernel module
HP-UX: NSAhttp
Linux: TUX
In-kernel real-time messaging server
Windows NT: http.sys
SPIN: http
OpenVMS: WASD.trap