A list of available D-Bus services

On QT setups (short commands and clean, human readable output) you can run:

qdbus

will list list the services available on the session bus and

qdbus --system

will list list the services available on the system bus.

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On any setup you can use dbus-send

dbus-send --print-reply --dest=org.freedesktop.DBus  /org/freedesktop/DBus org.freedesktop.DBus.ListNames

Just like qdbus, if --session or no message bus is specified, dbus will send to the login session message bus. So the above will list the services available on the session bus.
Use --system if you want instead to use the system wide message bus:

dbus-send --system --print-reply --dest=org.freedesktop.DBus  /org/freedesktop/DBus org.freedesktop.DBus.ListNames

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You could also use DFeet if you prefer a graphical tool (see the other answers for more GUI options).

The python way is the beautiful way.

System services:

import dbus
for service in dbus.SystemBus().list_names():
    print(service)

Session services:

import dbus
for service in dbus.SessionBus().list_names():
    print(service)

qdbusviewer is your best friend; it allows you to send D-bus messages as well: