memcached
memcached starts up with a block of memory it's allowed to use for caching, one that it manages quite efficiently. Like pgBouncer, incoming connections are handled using the libevent
library, so that any number can be serviced without timeout issues. The interface to the program lets you define key/value pairs of information that are then stored. Keys can only be up to 250 characters long, and values are limited to 1 MB. The program's popularity comes from how easily it can be used to cache content in a web site. Returning to the idea that the best database query is the one that doesn't have to happen, the fastest way to return a database-driven web page a second time is to send a cached copy of it. This avoids both the database query and the page rendering.
If you can figure out how to map your incoming requests into keys without spaces in them (which is easy for most web site URLs), and how to save the result into the amount of space the cache can hold, memcached...