This isn't data that belongs in a session.
I use globals as a mini memcache (at least I did with Lasso, and now I'm assuming I can recreate that with Ruby). It's a hash of data being stored in partially processed formats.
In my case I'll have a particular global dedicated to caching a particular type of data. The global will be created by the system that needs it, when it needs it. This way I can build sub-systems that create their own environment needs. If I need a db mapper, I install that. It creates it's own global for caching the various cross reference structures created from mapping definitions.
Example: I load value list definitions from a text file (or db, or even web service, source doesn't matter). Many value lists are stored in one file, or retrieved with one query. These definitions do not change, they're part of fixed options in the application design. I lazy load the lists as they're needed, parse each list into a data structure suitable for passing as options to a renderer, then cache each parsed list.
I have my own value list renderer class that provides some options and application-integration capabilities different than the Rails one. Some lists require on the fly additions or subtractions at various points in the application (modular app hierarchy, or permissions based adjustments). I can pull the base list from the cache and modify it in the controller before passing a var with the final list to the renderer in the view.
Also, I might go ahead and store a baseline HTML rendering of a value list in the cache, and then use regex to alter its final state. This is usually the fastest way to generate valuelists. On pages with a handful of them it doesn't matter so much. On pages with many of them or, several long ones, it can make a difference.
I know Rails has some tools for this, but IMO they're more cumbersome than my system, and they simply don't allow for some of the flexibility I need.
My using globals tends to be for application config/reference, so while using globals is good for creating machine-specific caches, I wouldn't use it for data where it would make more sense to use something like memcached.