Would cache in files be an alternative?
As anschauung mentioned you can use Boost to create a static page cache. Boost will copy the entire HTML of a page and store that HTML as a static file on the disk in a specific directory. A set of rewrite rules will cause the webserver to first look for a static HTML file for each request and serve that file if it's found. If it isn't, a normal Drupal request occurs via PHP. Requests served through the static file will be really fast, but PHP and the database are completely bypassed.

Boost is not a replacement for the cache backend and that Drupal will still use the cache tables in the database when a miss occurs on the static HTML files created by Boost. If you want to completely replace the cache backend, you could look into memcache, which stores serialized objects in memory. Memcached runs as a daemon on the server and can be connected to via TCP. This can be both a good and bad thing, depending on number of servers in the environment and 

Note that it is possible to have the database keep the cache tables in memory as well (at least with MySQL), although I'm not sure about the specifics of doing so.

You could all use the APC module if you're running Drupal 7. The APC modules integrates with the APC opcode cache and uses it as Drupal's cache backend. APC can be faster than memcached in certain scenarios because there is no TCP connection overhead. Each APC cache is tied to a specific server, while memcached and the database can be shared by multiple front end servers (again, this can be good or bad).