A few that I can think of: Although I'm not American, so I'm just going on what I don't see people post here.
1. Ezo Spruce (2 common varieties in Japan: Red and Black)
2. Sekka Hinoki
3. Kuchinashi (Gardenia jasminoides "Cape Jasmine")
4. Isozanshou - Also sometimes called "Tennouume/Tennoubai" (don't even know how this one would be translated, sort of a pepper plant native to Okinawa. They form little white flowers are grow peppercorns) Photos here
5. Japanese White Pine (some of you have them, but certainly not that many) - There are some cultivars I've definitely not seen on these forums.
6. Nishikimatsu (Cork Bark Black Pine? - Not sure if they're the same thing or not)
7. Toshou (Needle Juniper?)
8. Iwahiba (Selaginella tamariscina?) - Don't even know what these technically are. Not a tree really, but still many bonsai people keep them and display them, although not often in shows. (shrug) - They have hinoki-like foliage but appear to grow with a skeleton like a hard coral does.
9. Yuzu - Saw and amazing example at Seikouen last month. Rare as bonsai.
10. Haze (seems they might be called a wax tree) - Very common in small shohin/mame sized groups.
^#1-5 I personally have one or more of, and have not seem much of on these forums. The others I don't own but are common enough (minus the yuzu).