Microsoft offers some tools or build-in-features to make free/busy available. Basically GALsync does “synchronize objects” between forests. It takes users, contacts and groups from Forest A and creates contacts in Forest B. This is the basic requirement if you want to make free/busy information available between forests. GALysync works well in trusted and untrusted environments.
To have GAL in sync between forests GALsync is a good and flexible solution but offers no complex identity management features if this is required.
Getting GAL in sync between forests ILM 2007 may be a good solution but needs a lot of preparation. Also there must be a direct connection between the data sources, which often is unwanted by companies due to firewall restrictions.