Create a list of routing numbers and the banks they are associated with and include it as the standard table for Banks/Financial institutions. Since many organizations deal with many donors who deal with many different banks, one master list would allow for easier recording of what institution each donor uses.
It would be brilliant to have a version for UK banks, too. Maybe some kind of plug-in that would automatically update the latest info from BACS, and maybe offer the option to de-dupe the table too? The Table Cleanup function is a bit of a pain to do manually, as there can be hundreds of duplicate banks to get through...