Marcus Bointon

While it certainly helps an enormous amount on a single server, APC (and all other in-process caches) isn't a scalable solution because it can't maintain cache coherence beyond a single host. As soon as you have more than one web server, you need to use something distributed such as memcached or redis.
For front ends, both nginx and haproxy are faster than varnish and will scale to higher connection counts. They don't necessarily do the same thing, but there is a lot of overlap for proxying, balancing, failover, caching, SSL unwrapping etc that they can make a better job of than varnish.