Bear with me, I'm trying to keep this as general as possible so that I don't violate any copyrights. (It's not SRD material.)
You got Move, Standard, Swift and Immediate actions, right? Tome of Battle uses this to the fullest in combat. It uses a system called manouvres. A class that prepares these is called a Martial Adept. (This is not a class per se, but a general term, like "Spellcaster.")
Each martial adept has a number of manouvres known and readied. (See link at the end for a sample Martial Adept, called the Warblade.)
Manouvres come in three type: Strikes, Boosts & Counters. Then there are Stances.
Stances: These can be active as long as you like, and give you some general bonus, for example: Blindsense or so.
Strikes: These require a standard or a full action to initiate ( "cast", if you will) and generally require and attack roll and they do weapon damage. An example: A strike that lets you treat the attack as a touch attack.
Boosts: A swift action to initiate, these give some bonus for the rest of your turn, for example: 1d6 extra fire damage to all attacks.
Counters: An immediate action, these are used in reaction to actions by the enemy, for example: After an opponent attacks you, you can initiate this to make an opposed attack roll and if you score higher, you parry the attack and take no damage.
The number of manouvres readied is per encounter. Each encounter, you can use each readied manouvre
once. The Warblade, for example, can make an attack and spend his swift action to regain all his readied manouvres.
In between encounters, you can change which manouvres you have readied, based on the ones you know.
The catch is that by using the different kinds of action, that you are forced to think ahead. "Should I spend my swift action to use an immediate action and use a Counter? Or should I wait and use it to power a Boost?"
Different Strikes have different effects: It depends on you to maximalise them. Since most Strikes are standard actions, people tend to stick much less to their full attacks, creating more opportunities for movement and thus avoiding a slugfest, which boils down to: "I attack. I do damage."
On first glance, you might think that the system is overpowered. It isn't. Sure, the Warblade is better than a Fighter. Which class isn't?
Tests on the Character Optimisation Boards on Wizards have proven that, for example, a Warblade doesn't do more damage than a Barbarian.
Ofcourse, there are sick broken combo's possible, but then again, that's also possible in Core-only. (Natural Spell? Time Stop? Gate?)
Here's the link:
Tome of Battle: The Book of Nine Swords