Subsystems

Bus (PCI/USB/SBUS/ISA/SDMMC/I2C/GPIO/...)

Device Class (SCSI, ATA)

Networking Layers
net: generic networking goo
netinet/netinet6: TCP/IP & IPv6 implementation
net80211: IEEE 802.11 wireless stack
netbt: Bluetooth socket interface
netatalk: Apple Talk
netnatm: ATM bits
netipx: Novell's aging IPX protocol

Higher level
wscons: input and display abstracting/multiplexing
bio: kernel block I/O storage abstraction
tty: terminal handling
sensors: nice abstraction to sysctl for reporting values
audio: make it easier for audio drivers to play humppa
radio: sound/video tuners
ifmedia: handling network interface options

Interfacing to a kernel subsystem normally takes the form of filling out a few
structures and perhaps callbacks in attach code then using provided functions.