In our instance of Salesforce.com, we have a custom object called Projects. Within a Project, we have various opportunities for our multiple companies.
I would like to display the Opportunity Probability for each company in Project fields. I assumed that I could use Roll-Up Summary fields to summarize this Opportunity information, but there is not a Master-Child relationship between the Project object and Opportunities.
My thought is that we could create an Apex trigger that would populate the Project fields on Insert, Update, Deletion of an Opportunity (and anything else that I may have missed) to the project. The trigger would need to loop through all opportunities on the project and (for now) determine the MAX probability for each company (Company is a picklist value on the Opportunity) that do not have a Closed status. After determining the MAX values, it would populate corresponding Project object fields with each company's value.
It's possible that we may want to change from using a MAX value to something else (simple average, etc.) in the future. Other changes may be necessary as well (e.g. only using values from Opportunities of certain statuses), so we'll need a little bit of flexibility in the Apex to change it down the road.