I like ornamental grasses.

Something to keep in mind is that unless restrained, a mature baptisia is a large, sprawly plant that covers a lot of real-estate. Restrained, it behaves like a fountainy shrub with a noticable heavily shaded area at the base.

So, for grasses, to compliment the shape, you'd go more draped. Like a Prairie Dropseed or a perennial Fountain Grass as opposed to a Karl Foerster? What about smaller varieties?

I have Amsonia hubrichtii growing next to/in front of mine. I like the textural contrast (ornamental grasses would offer the same) and the blue star flower compliments the blue-green foliage of the baptisia. Mine is the hybrid - Twilite Prairie Blues - so not quite as massive a plant as typical baptisia. Plus, the combo looks stunning in fall.

Like Gardengal, I have Amsonia hubrichtii with several of my Baptisia. They have a relatively short bloom time, so I wanted to have foliage that looked good with Baptisia, both color and texture.

Here's Baptisia 'Briggs Moonlight' with Amsonia hubrichtii.
{{gwi:263536}}From June 5, 2012

And here is a species B. australis with Amsonia, blue spruce, and Coppertina ninebark burying a gold hosta (that would have been moved had the voles not eaten it.)
{{gwi:263537}}From June 5, 2012

I have two false indigo plants growing in my butterfly bed, with lots of other full-sun perennials growing around them, including Shasta daisies, Echinacea, Nepeta/catmint, Rudbeckia, Caryopteris/blue mist shrub, rose of Sharon, peony, & butterfly bush.

My artemesia has gradually diminished over the past half dozen years so I no longer count on it returning every year while cushion spurge (Polychroma spurge euphorbia) has been with me for more than 25 years.

A low-growing option that requires zero care is Stachys 'Helen von Stein.' It's a sterile (without the ugly flower stems) lamb's ear cultivar that just looks great 11 months out of the year.

I thought I had a better photo of an earlier season, but I didn't have time to look longer. If I would just label my photos consistently so they would all come up in a search, I'd make my life easier. But this is Baptisia australis with Nepeta 'Walker's Low', two varieties of Cranesbill and Ninebark 'Summer Wine' which I think is the same as Babs. It only gets less than 5 hrs of sun in the afternoon here and it needs support because it flops, I assume with not enough sun. Not sure I'm loving all that purple color.