There used to be many model-caching plugins for Rails, but few if any made the transition to Rails 3. The record-cache gem looks interesting and flexible, though I haven't tried it:
http://api.rubyonrails.org/classes/ActiveRecord/IdentityMap....
https://github.com/rails/rails/pull/5261
Though there is now a company volunteering to fund completion of the feature, starting Real Soon Now:
https://github.com/rails/rails/issues/5442
IdentityMap would, obviously, be great. Though I didn't realize it could serve as a replacement for model caching; didn't IdentityMap, like the query cache, only last for the lifetime of a request? Or could it be persisted in the Rails.cache of your choice for an arbitrary lifespan?
IdentityMap will be extremely useful for the majority of sites since they have very low traffic, processor, and memory demands, and therefore it's unlikely you'll have too many processes running using up their own memory containers. Though you could argue these sites really don't need additional caching anyways (they'd still respond slightly faster with IdentityMap).
For high traffic sites it'll still be useful to save some trips, but you'll also want to implement and manage a distributed cache (for models, actions, fragments, etc).
It's available and sort of works now. You just have to turn it on and be mindful of how associations are handled. Still a work in progress.
However its biggest drawback is that its hard-wired to use memcached.
It would be great if the actual cache backend was pluggable and we could develop a redis adapter. As it stands the memcached integration is pretty deeply intertwined into the cache-money library.