NARGS member Tim Ingram mentioned his interest in Lomatium, a North American genus of umbellifers. See: http://nargs.org/smf/index.php?topic=662.0;topicseen
The plant family Apiaceae is huge, with 347 plant genera, with many species of interest for rock gardens, so let me start up this topic with links to another North American genus, Cymopterus.
I share an enthusiasm for rock-garden-sized Apiaceae, and have long admired both Lomatium and Cymopteris when I've seen them both in photos and during my Western American travels years ago... some are truly superb. Here are some selected links to photos of Western American species of Cymopterus.
fruiting heads on Cymopterus cinerariushttp://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0908+0697 flowering Cymopterus cinerarius, fantastic foliagehttp://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0908+0696Cymopterus gilmaniihttp://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+1210+1537
Cymopteris globosushttp://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0306+0913Cymopterus multinervatushttp://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+1004+0875
Some Apiaceae that I've seen photos of, in places such as Kazakhstan, and in New Zealand and Australia, show these are fantastic foliage plants, with strangely beautiful flowers.