At least the venerable /me.
We should make it clear in the UI when a command is being executed, as opposed to a message being sent. We can e.g. change the send button color and icon. Also we should allow // to escape a leading slash.
The send field should also be considered empty with regard to typing notifications when the field starts with / (eg if you start typing /join #foo, there's no need to send out @+typing since you're not writing a message to the channel)
Simon Ser referenced this ticket in commit 53ff834.
/meis now supported.