An great post on creating relationship lists in SharePoint.
I have been working on a 2010 project to deliver a tracking system inherited from a spreadsheet. Very standard SharePoint stuff here. In 2007 we would have had to implement either a one way link from child to parent or parent to child; or two links both ways but totally independent.
With relational links in SharePoint 2010 we get a much richer tool for creating links between list items. In this way you now can create proper parent child relationships between lists in SharePoint 2010.
When you create a lookup link column in a list, you now can impose relationship including cascading deletes. It also allows lists to inherit information from other lists via a parent child relations.
How we are using it is linking a task list item to its review parent. The review parent contains key information needed for doing the task, and now we only need to link a new task to its parent. Planners using SharePoint create tasks and assign them reviews that identified the issues in the task. It is a vast improvement over the first version with did in 2007.
What I have not gotten a chance to test yet is if this also can work across web applications.