I've recently switched from Keyclone to Inner Space.
I know people mouse broadcast successfully with HKN as well.

Keyclone is a great program as far as key broadcasting goes.
It has great support and decent functionality.

Inner space has the same level of support.
But it also has automatic FTL (via a wizard), strong mouse broadcasting and near enough to instant PiP swaps.

I've not played with HKN at all, so cannot really comment on it, although various boxers swear by it too.
HKN uses scripts, which I'm not totally sure how to play with, but you can research them too.
I believe HKN is a free program.

Inner Space is $36/year for up to five computers. There is a reduced rate for a shorter period. And you can get a free trial too.

Zanthor has a sticky post, "From Zero to Hero in 30 Minutes" or something with a very similar name.
He describes the mouse broadcasting in a lot of detail towards the end of his guide.
The IS Wizard now sets up more then 90% of what his post covers, but its still a good read as it explains how you would manually do what the wizard does for you. Which greatly helps when you want to make your own system.

Most of the set up takes place with a program called ISBoxer Toolkit.
Once you're done there, you export the changes to Inner Space.

There's an option in the toolkit for a hotkey for the in game GUI.
Press the hotkey to open the GUI.

There's a drop down box where you pick which clients will duplicate the clicks from the current wow window.
I believe my options are 1, 2, 3, 4 or All Others (I five box).
Once you choose an option, click the arrow thing which makes a box appear on the screen.
Drag the box where you want it and pick a name for it.
Then save it.

This is a "mouse repeater region", which means any mouse movement or click (including modifier keys such as Alt, Shift, Control) that takes place in this region is passed to the preselected other clients in the same position.

So with an addon like Open RDX or Vuhdo or Grid (with GridCustomLayouts), which can list your party or raid in alphabetical order, instead of your name and then others names, will let you broadcast to the desired target.

I'm personally using Grid + GridCustomLayouts. I have a window which lists my five toon's names in alphabetical order. I've set it to display this list when I'm in a party or a raid. If i want, I can type /Grid Config and then click Layouts and pick the default option for raids, to be able to visually see everyone in a raid. At the moment, my group only cares that my priest can heal my own members.

So on all screens I have the same Grid layout, which lists my five team members alphabetically. I have this repeater region created on four of my five toons (not on the priest, as I'd just heal from that account and not need to pass a click to anyone else). No matter which account I'm on, it only passes the click to the priests account, since I've configured it that way. I have also checked KVM mode, which means the click does not apply to the window in which I'm clicking in.

Then the Clique addon is pretty simple. There is an extra tab on my spell book, which opens a window beside the spell book. When this extra window is open, and click or combination of clicks done to a spell is recorded. I've recently experimented with my mouse side buttons (thumb forward and thumb backward). So far I have Renew, Penance, Flash Heal and PW: Shield on binds.

From any window other then the priests, I can click on my mouse repeater region and have the same click broadcast to the priest. So an Alt Left Click will renew, while a thumb forward button will cast Penance. I like to not have anything on a no-mod left click, so the priest will still be able to select a toon to say open a trade window, without casting on that toon instead of targeting them.

The strength of the mouse repeater region is that clicks are automatically broadcast when within the window, but I don't need to turn mouse broadcasting on. With InnerSpace, you can enable mouse broadcasting, so any click anywhere (or for that matter, mouse movement) is broadcast to all clients. I used this for clicking pumpkins on all toons at once, earlier today. However, most of the time I won't want total mouse broadcasting (repeater mode) to be on, as that would make simultaneous right/left button presses for movement awkward.

Just moving to the repeater region and it turns broadcasting on automatically. Move the mouse out of the region and its off. Incredibly fun and powerful.

I highly recommend InnerSpace.