[jikbd]

Automattic, the makers of Wordpress, have a free and very powerful cache plugin.

[danmur]

It's just not the fastest cache plugin (at least by name)

[robjan]

It's "good enough", in my experience, and works on nginx and Apache with minimal configuration.

Some plugins break it and the object cache leaves a lot to be desired but for a basic wordpress you can pretty much activate it and forget it.