Hi -
"lspci" is handy for PCI devices.
"lsusb" is the equivalent for USB devices:
http://linux.die.net/man/8/lsusb
PS:
The trend for all devices is hot-pluggable/dynamically configurable. From that perspective, asking "What devices aren't plugged in?" is much like asking "What is the sound of one hand clapping?". To know, is to achieve Enlightenment ;-)