I wonder if hosta or daylilly growers and collectors call all the other plants they grow for companion plants too? For rosaholics all other plants are just that - companions. Roses are the Queen of flowers, but a Queen is nothing without her companions - and to be honest I think a garden with roses alone is a bit boring. So I also collect hardy geraniums or crane bills. I have many different types, but prefer those with a long flowering time.
One of my favorites are Geranium Cinereum 'Ballerina'. It is an alpine type geranium, that forms small mounds about 6 inches tall, and flowering from spring to late fall in flushes. Like other hardy geraniums the plant look more tidy if trimmed down each time it has flowered. It prefers a vell drained soil, but tolerates anything but poorly drained clay soil and are hardy down to USDA zone 5. It spreads slowly compared to other hardy geraniums and may only need to be divided after 3-4 years. It looks nice in the front of mixed borders. It is so elegant - the name ballerina very fitting.