Are there any lists of shrubs/small trees that are best viewed from below, such as Styrax or Halesia?
While there are no lists of shrubs/small trees best viewed from below, there is a list of trees with weeping habits in The Pacific Northwest Gardener’s Book of Lists (Ray and Jan McNeilan, 1997). Many genera of conifers – Cedrus (cedar), Chamaecyparis (cypress), Larix (larch), Picea (spruce), Pinus (pine), and Tsuga (hemlock) – have weeping forms, often indicated by a variety name ‘Pendula’ or ‘Pendulum’. There are weeping birches (Betula), beeches (Fagus), and cherries (Prunus), too.
You are correct about Styrax and Halesia. Additionally, I ran across a few individual species that may be of interest to you as I researched this question:
–Davidia involucrata
–Laburnum anagyroides
–flowering currants, Ribes spp.
–flowering cherry trees, particularly Prunus padus
–various plants in the Ericaceae family have bell-shaped flowers that hang on the underside of the stem.
I would add that any tree which has a naturally graceful branching pattern and/or delicately shaped foliage (such as Japanese maples) would be pleasant to view from below, as well as from other angles.