From Infowise Ltd
You can now create cascading master-detail relationships between two or more lookup fields in SharePoint lists and document libraries. Filter your city column values by the selected country, it is as easy as it should be!
* Support for site columns and content types - now you can create the master-detail relationship at the root site and reuse it throughout your site collection.
* Support for multi-choice lookup fields - both master and detail fields can be set to support multiple values. When several master values are selected, the combined list of detail values is available for selection.
* Unlimited number of cascading levels - you can chain together as many columns are you like, each parent can have unlimited children and each child can have its own children. Note: you may experience some performance degradation when too many columns are added. Up to 12 columns may be created on a slow system without a significant performance penalty.
* Looks and behaves similarly to the built-in lookup column, no user training required - end users will not notice any change in the behavior or appearance.
* Enter new values directly from the column
* Operates both as a cascading lookup and as a regular lookup
* Supports filtering and sorting values according to a pre-selected SharePoint view
* Add addtional columns from the source list and update their values automatically
* Based on ASP.NET 2.0 AJAX capabilities.