The Boeing EELV

In the mid 1990s, this publicly-unnamed rocket was Boeings bid for the Evolved Expendable Launch Vehicle Program. It featured 2 RS-25s on a reusable engine pod, a variable number of strapon SRBs, and an RL10-powered partially-hung upper stage comparable to the Delta Cryogenic Second Stage. Ultimately they lost the competition to Lockheed Martin and McDonnell Douglas, then merged with McDonnell and stayed in the EELV program using the latter's bid, Delta IV, so this vehicle never flew.

Kind of an ugly design, but technically pretty neat. This should have offered performance comparable to DIVH, but at a small fraction the cost thanks to partial reuse, fewer configurations/unique elements, and engine commonality with the Shuttle.