My favorite is Euphorbia 'Diamond Frost', which offers a delicate, fluffy contrast to the heavier texture of the pellies and a nearly continuous bloom from spring through frost. But this is not available from seed to my knowledge.

Otherwise, trailing lobelia is a popular choice but prefers cooler conditions and may not last a full summer if in a hot and dry location. Lobularia maritima - sweet alyssum - is another popular choice. Or Chaenostoma cordatum, aka bacopa. All can be grown from seed quite easily.

Are we talking about Geraniums or geraniums (Pelargoniums)?

I meant Pelargoniums (I think!) -- the tender one in garish colors that is the only thing that i've been successful with in my summer containers. I looked up each of gardengal's suggestions and they seem to all need more moisture than the geraniums I'm thinking of. I did pair them with trailing blue lobelia last summer which was attractive until it wasn't.

Ok, gardengal, I will take your advice and try bacopa and euphorbia this year. Know if they can be wintersowed here in zone 6b? Thanks.

Can't help much with that - I don't do seeds :-) For a few containers, purchasing starts seems far easier and less expensive in terms of time and effort that starting seeds. And the euphorbia is not available from seeds anyway.

Hmm... I just ordered seeds for Euphorbia marginata (Snow on the Mountain) from Sampleseeds, which says they are good for winter sowing.

Different plant entirely and will not give the same fluffy, airy effect as the Diamond Frost. Diamond Frost is a hybrid and is sterile.

So you said. OK, then I will wait to buy starts in early spring (they seem commonly available) and follow advice elsewhere here to multiply by taking cuttings.. Thanks for the correction, and showing us the difference between great gardeners and the rest of us:)

I often use the diamond frost euphorbia with geranium, too. Dusty miller looks good, asparagus fern (a plumosa) provides great textural contrast. Don't forget petunias, especially the "carpet" varieties.
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Al