My personal favorites are dahlias, crocusmia, phlox, hydrangeas, and astilbe.

One of my favourite fillers for vases of roses is "Artemisia Lactiflora Guizo" (white mugwort). This is a clump forming perennial with evergreen foliage. It sends up 3-4 foot branches of white flowers similar to Baby's Breath, but with more substanital branches. It is long lasting both in the garden and as a cut flower. The clumps expand fairly quickly, but it's easy to divide. Many visitors to the garden go home with a small clump.

Here is a link that might be useful: Garden Splendor

What I like is more roses. Little floofy ones in neutral colors, white, pale pink, old-fashioned red. They make great filler, and provide a nice contrast to large angular hybrid teas. I also like shasta daisies and ferns of all sorts. There are a lot of wonderful salvias--some come in true blue!--that provide nice fluff. I have Japanese Anemones too that make great cut flowers, and they bloom in autumn when not much else does.

I love to add angelonia & pentas (annuals), roses like the Fairies with small blooms and bushy foliage, verbena, perennial geraniums, phlox, salvia & sage, hydrangeas, lantana or just about anything is game if its blooming or "living" in my yard. Even things with pretty foliage without blooms. I like to play with all kinds of plants and foliage.

Blue cloud larkspur, ammi majus, ammi visnaga, snapdragons, orlaya, different forms of feverfew, astrantia, asters, delphinium.

The Green Rose is perfect in arrangements. It's adds a wonderful texture.

If the rose is not fragrant, add rosemary, which is fragrant.

Lavender is also nice and will suffice.

I try to arrange my roses and make bouquets, but things always turn out a bit off. My usual filler of choice is more roses ... cluster-flowering ones with smaller flowers like polyanthas or noisettes. The Noisettes smell wonderful!

Connie

Ming Aralia is my favorite. It is a house plant in most places, here it grows outside beautifully to 6'. It has lovely lacey foliage. I also like ivy, which lasts for a month in a vase.

I like to go out into the garden and cut my roses and just walk around picking different things to add to the bunch. Grasses can be nice when they are in bloom, or the striped ones. Also like the ferns, salvia, lavender, tall straight plants with a nodding bloom is pretty too.

Among my favourite fillers are lavender, perennial baby's breath and violas with smaller white and pink roses. Red is a difficult colour and I find that the lime of alchemillas goes well with even the brightest red and tones it down so I can add white if I want to. Red and white only together is a too harsh combination. Asparagus sprays are a nice fluffy filler.

I second all those who said... more roses! LOL
Especially those with frilly or smaller petals that don't look like your traditional HT's will really complement them. My favorite for uniting random colors in a vase is Cinco de Mayo, which is weird because it doesn't match anything but works with everything. Also Abraham Darby works with everything and smells wonderful.