Robert A Curedale has written in 2013 a book called "Service Design: 250 essential methods" which offers one of the biggest overviews of Service Design methods I've seen so far.
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The website Service Design Tools is one of the references for finding a list of Service Design-related methods. It also has a pretty interesting section called Enhanced Tools. There you'll find methods that avoid some of the weaknesses of old school Service Design work.
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There is also the This is Service Design Doing Method Library with 50+ methods with each a pretty good description. If you like those, you should check out the book This is Service Doing.
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The design team behind Capital One has created an illustrated poster of Service Design Tools & Methods.
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If you want to go beyond Service Design, I have a list of "Methods Libraries" that you might want to check. These are websites that list design, innovation and other methods that can be helpful.