There are many different ornamental onions and most of them make good cut flowers but I only regularly grow two. The first is Allium hollandicum 'Purple Sensation'. The flower is a medium purple 2-4 inch sphere that is actually composed of many tiny flowers. There are many other hybrid alliums like 'Gladiator' or 'Globemaster' which produce huge 6-10 inch spherical flowers. These can look kind of fun in a perennial garden but are too large and expensive for me to use in my arrangements.
'Purple Sensation' alliums with lilacs, coral bells and double columbine.
Bulbs for 'Purple Sensation' are planted in the fall and bloom the following year in mid to late May. The first spring each bulb usually produces one large flower and in following years the flowers will be smaller but more numerous. After a few years you can divide the bulb in the fall and re-plant the pieces to propagate it, but the bulbs are cheap enough - around $35 for 100 in 2007 - that I have never bothered.
The other allium I use is Allium sphaerocephalon, or drumstick allium. This allium is smaller and dark rose-purple colored. The flowers appear in July and are shaped like little 1 inch eggs on top of 2 foot stems. I don't use this flower as much as I used to just because there are usually lots of flowers available in July.
The cut stems of these alliums will smell a little oniony, but the flowers themselves are not pungent. This is not true of all of them. I used to grow a third one called Allium bulgaricum. It is sometimes listed as Nectaroscordum siculum. It is really striking with clusters little crimson bells each edged with pale yellow. You can see them in the picture below in the center and upper right. These are really pretty and I would still use them except that when the stems are handled they give off a really strong skunky garlic smell. The smell fades after a bit but I decided they were too risky to send into someones house. They are not expensive and would be great in a perennial border. I'm sure that deer and rabbits leave them alone.
Iris, coral bells, and that cute little stinker Allium bulgaricum