FarCaster is an addon primarily for healers or anyone who wants one-click buttons for buffing/debuffing. It works similarly to BeneCaster (which was based on Caster), an addon which is no longer updated. Note, FarCaster is NOT technically a continuation of these addons. It's completely rewritten from scratch, and none of the original authors are involved in this.

Basically, FarCaster lets you select beneficial spells form your spellbook and have them attached to player, party, raid, target, focus and pet frames. You can change the order of the spell buttons, and can freely position the row of spells in relation to its unit frame. Dual spec is supported. In fact, you can respec back and forth and FarCaster will be able to remember your setup, since each tree is saved separately.

User guide:

Important: FarCaster will not start working until you have at least one talent tree.

Click the minimap icon to open the options panel. First click on the unit frame type you want to add spells to. Then select spells from the list on the left. These spells will appear in the list on the right, and you can move them up and down to change the order. Use the slider to change button size. Enter positive or negative number values in the X and Y edit boxes to control the placement of the buttons.

Buttons will be marked red if target is out of range, blue if your mana is too low to cast spell.
Spells that are already applied to target will be faded out until the buff is gone.
If a debuff that the player can dispell is present, then the spell button will be colorised according to the debuff type.

It is possible to force a redraw of the spell buttons by pressing a key, which must be set in Key Bindings found in the standard WoW option dialog box.

Unit Frames support

FarCaster has been tested with the following unit frames:

Default Blizzard frames (patch 4.0 style)
X-Perl
PitBull 4
Shadowed

There is theoretical support for Perl, Discord, AG_UnitFrames and PitBull since the frame names have been copied from BeneCaster, but these have not been tested. Support for more unit frame systems depends on interest and feedback. If anyone wants to test this addon with other unit frame systems and report their findings (error messages, etc) then that would be very helpful!

Troubleshooting:

The Blizzard API does not allow buttons to be added to units (i.e. players, targets) that are in combat. Thus, when someone joins your party/raid in the middle of a fight, you may experience missing button rows and other anomalies. The solution to this is to manually force a redraw of the spell buttons when everyone is out of combat. This can be done via a keyboard shortcut which must be set in Key Bindings found in the WoW options panel. If all else fails you can reload the user interface by using the slash command /RELOADUI in chat.

Finally:

Any sort of feedback, suggestions and criticism is much appreciated. :-)

A big thanks to the people who made Caster and BeneCaster for coming up with this idea in the first place!