Lookup to lists across sites, even site collections with the Bamboo Lookup Selector custom column.

The Bamboo Lookup Selector combines the features of the popular Bamboo Selector and Bamboo Linked Selector, and still allows users to easily and intelligently reference existing data in your SharePoint lists, access list items from across your Site Collection, and add automatic duplicate removal and find-as-you-type functionality to your lookups. You can also add parent-child relationships to your columns, allowing the data entered in one field to determine the options presented in another.

For more complex data entry, simply select the Allow Multiple Values option, and the front-end UI changes to allow multiple selections to be added or removed from a list -- choose as many as you'd like. If you're using a series of parent-child Lookup Selectors, each subsequent field will immediately update as you add or remove each selection from the prior one.

And now, the new Lookup Selector goes even further—adding automatic updating in response to changes in source data, ensuring that your Lookup fields aren't populated with out of date information. If you change information on a source list, any Lookup Selectors tied to the data will update automatically.