Ultimate Relay Team Generator.
This is still a bit scruffy, but on the whole it works.
Choose the age range, the event and the date of the race and it will select a list of candidates for you and, for IM, select the best combination of swimmers in each leg. You can manually exclude swimmers from individual legs if you like and it will update the best team.
The times used are all 50m PBs converted into short course times using the approved ASA conversion algorithm.