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 cinerarius
http://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0908+0697
flowering Cymopterus cinerarius, fantastic foliage
http://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0908+0696
Cymopterus gilmanii
http://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+1210+1537
Cymopteris globosus
http://calphotos.berkeley.edu/cgi/img_query?enlarge=0000+0000+0306+0913
Cymopterus multinervatus
http://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.