nginx / ngx_supervisord (project fully funded by megiteam.pl)
Module providing nginx with API to communicate with supervisord and manage (start/stop) backends on-demand.

As a "side effect", it also provides a way for dynamically taking backend servers out of rotation.

View:
README file,
CHANGES file,
APIv2 specification.

Download:
ngx_supervisord-1.4
(SHA1: e61f09244d30f3652f7276088470be2031b3666e)

GitHub repository:
http://github.com/FRiCKLE/ngx_supervisord/

Example configuration:
upstream backend {
	server 127.0.0.1:8000;
	server 127.0.0.1:8001;
	supervisord 127.0.0.1:9001 admin:super;
	fair;
}

server {
	location / {
		proxy_pass http://backend;
	}
}