Object_space_profiler is a project mainly written in Ruby, it's free.
Periodically profile all outstanding Ruby and/or ActiveRecord::Base objects.
This is just a quick hack. YMMV.
Drop active_record_memory_profiler.rb in somewhere it will be loaded in your Rails app, and then call ActiveRecordMemoryProfiler.start. It will log all outstanding ActiveRecord::Base-derived objects to log/ar_mem_profiler.log every 10 seconds. See source for config options.
Drop object_memory_profiler.rb in somewhere it will be loaded by your app, and then call ObjectMemoryProfiler.start. It will log all outstanding ObjectSpace objects (not including strings by default) to log/obj_mem_profiler.log every 10 seconds. See source for config options.