It annoys me no end that the LMB is for the right hand and the RMB is for the left hand. Is there a mod or console command to swap them for combat only (i.e. not for menus, dialog, etc)?
Well, just go in your menu (Esc if on computer), Controls, and switch the buttons for Right Hand and Left Hand, and voilà
It's not as easy as you guys make it believe. If you switch the mouse buttons, you ARE going to get in trouble with menu's. But by this time there's a mod that exactly addresses this most annoying thing. It's called: On-screen Interface Key Fix, and I use it myself with joy. You can change whatever you want but the keys in menus will not be affected.
It can be downloaded for free and without subscribing at skyrim nexus.
Another key mod at skyrim nexus is called: Interface Hard Coded Key Tweaks
It comes with an optional file called "skyrim key helper", which can also be downloaded from steam, but for that you need to subscribe to steam first [no surprise].
As Mikalichov says, it's easy to change in the menus on PC. I was irritated by this too and made the switch early in the game (around 20 hours, yes that's early).
This is only for combat. It doesn't effect menus, alchemy, enchanting, smithing or trading at all.
But there is not option to change the "preferred hand". For instance a single handed weapon always gravitates* to your right hand and spells/shields gravitate to the left hand. This happens even if you switch the mouse buttons and as far as I can see, there is no way to swap this.
If I'm wrong, PLEASE say, because I may be right-handed, but I'm index-fingered (that sounds bad).
*When I say gravitate I mean when using the favourite buttons 1-8.