The Robot Co-op, the team behind 43things, has released several great Ruby libraries for free, all available as gems. Here are some of the highlights:

cached_model: A subclass of Rails' ActiveRecord::Base that caches objects in memory using memcached. Big speed improvements in many circumstances.

geocoder-us: A library that accesses geocoder.us's geocoding API that lets you look up addresses and turn them into coordinates.

google-geocode: A library that uses the Google geocoding API to the same effect as the previous library.

memcache-client: A pure Ruby client for memcached, the great memory cache system.

yahoo-search: Bindings to Yahoo's search API.