If you find yourself developing or debugging D-Bus services or applications which use them, you might find some of these tools useful.
Calling remote APIs
dbus-send, distributed with D-Bus, allows you to invoke methods on services from the command line.
qdbus, shipped with GLib and Qt respectively, provide (arguably) nicer command-line syntax and output for essentially the same task.
Monitoring bus traffic
dbus-monitor, distributed with D-Bus, prints out traffic on the bus. You can filter the output by passing match rules as arguments.
gdbus monitor subcommand lets you monitor a particular remote object.
Shows the object hierarchy exposed by running services, allowing you to call the methods you see. More information.
Records D-Bus traffic (like dbus-monitor) and shows it as a sequence diagram, with built-in filtering and statistics. More information.