I spent a weekend looking into this, so I might as well post a reply to this old, old thread.
These are currently the best Aqua themes for:
XMMS: Winamp X (XMMS version) v1.01 (from the XMMS Skins list
GAIM: Aqua v0.1
GKrellM: AlmostX Gkrellm
KDE: Aqua OS K
If you use a window manager other than KDE or MetaCity, then you should have no trouble finding an Aqua theme for it. After all, most window managers have several Aqua themes available.
Glacier is not an Aqua theme, but it is a MacOS theme, and it looks similar enough to the other Aqua themes to look good beside them. There are GTK2 themes that mimic the Aqua look more closely, but they all have problems with either X-Chat (by turning the input box white) or Thunderbird (by leaving parts of the main window gray when they should look white). Glacier has none of these problems. It does leave the file download dialogue boxes in Mozilla Firebird unchanged, but this is much less annoying than the problems the alternatives have.