This is kind of a weird one, but I have cyprepedium (lady slipper orchid) growing under a butterfly bush. Also, alchemilla (lady's mantle). They both do fine.

Eleanor

I have sweet woodruff rambling through. I love the look but for some it is invasive.

This kind of thing takes a while to develop, both as an idea and as a reality. Here's what I'm trying so far... some aren't shade plant under sun plant, but simply tall plant over short plant. Maybe I should have asked about 'double-decker' planting?

Asclepias incarnata over Dianthis bath's pinks.
Violets under Gypsophila.
Muscari under Buddleia.

It seems that the ground could be completely covered by plants, eliminating the need for mulch.

Heucheras, hardy Geraniums.

I planted impatiens under lilacs last year, they did great. But they're annuals so I need to get some more for this year.