The 7th level Sor/Wiz spell Energy Transformation Field from SpC is one of the most powerful spells available. There's a good reason why it's banned in many games or contests such as the Test of Spite. However in a game world where it's available, there are many ways to take advantage of it in theoretical optimization.
Energy Transformation Field allows you to store any spell within it (which is presumably cast into it at the same time the field is cast, similar to Contingency). Afterwards the field will absorb all magic in the area (including spells cast, Sp and Su abilities used, and magic items activated) and use those spell levels to power off instances of the spell stored within. Because the spell is Permanent duration, effectively you cast a spell once, and can reproduce it an infinite number of times. You don't want to cast your own limited per day spells to power it, but rather you want to use an at-will spell-like ability, or a freely activatable magic item like the Immovable Rod which adds two charges each time you click it (because it's based off a 2nd level spell).
There are no limits on the "single spell linked to it" that can be put into an Energy Transformation Field (other than it can't have XP or expensive component costs). It says the spell is "cast at a point within the field designated by you", which implies even Touch or Personal range spells can be associated, which get cast upon whoever is at the appointed location when the field’s spell is activated. This makes Energy Transformation Field one of the few ways to get Personal range spells cast on other targets. Other ways include the Share Spells ability for familiars and animal companions, Glyph Seal magic items from MIC, Bracelets of Spell Sharing magic items from DMG2 to cast on you and a partner, and Shapechanging into a Symbiont and latching onto your target to share spells with them.
There are also no limits on the level of spell that can be put into Energy Transformation Field. Go ahead and use your metamagic reducers to put in an Extended Persistent Shapechange. Then have an army walk through while you click your Immovable Rod to give each person a two day Shapechange. Give the enemy army quite a surprise when all your troops suddenly turn into something very nasty! The only limitation is that (like Contingency) all spell choices are decided ahead of time. Hence you can't place a spell like an illusion (unless it's always the same illusion), or Polymorph Any Object (unless you are always polymorphing targets into the same thing).
Clicking an Immovable Rod will take 5 clicks to activate a 9th level spell. A Ring of Telekinesis (based on a 5th level spell) will add 5 charges and hence take 2 usages to activate a 9th level spell. However the fastest way to activate Energy Transformation Field is with a magic trap. An automatic resetting trap of Magic Missile at 1st caster level costs 1200 gold to buy. (It costs 500 gold and 40 XP to craft, where 500 gold x 2 + 40 XP x 5 = 1200.) Create 9 such traps, each attached to a lever with a larger lever or floor plate that activates all nine at once. Place it in the field, and you can walk in and as for what amounts to a move action activate all 9 traps and all 9 spells, and hence cast the associated 9th level spell for free.
Energy Transformation Field is stored at a fixed point in space, which normally means you can't take advantage of it while adventuring unless you teleport to and from it. To get around this, cast it inside a Bag of Holding, so you can carry it with you, and more importantly be able to close the bag to effectively turn it off when you want. Stick your 9 traps of Magic Missile on a small item inside the bag. Then you just open the bag, reach in and press the button to activate the traps, which get transformed into a 9th level spell cast for free. A good buff choice is Maximized Time Stop. Each time you click, you get 5 free rounds of time. Click the button once for 30 seconds of free time, or click it 120 times in a row for what amounts to an hour long Time Stop.
Fields allow a Sorcerer to extend their spells known relatively cheaply, to help overcome their Tier 2 limitation. A scroll of Energy Transformation Field costs 8525 gold (2575 for a 7th level scroll + 5000 material component + 250 x 5 XP). A Bag of Holding 1 costs 2500 gold. A custom Runestaff of a 9th level spell (and a throwaway 1st level spell since Runestaffs must have 2-5 spells attached to them) costs 16300 gold. This allows a Sorcerer to be able to cast a 9th level spell for 27325 gold (and without using your own spell slots). Actually cheaper than that, since you can sell the Runestaff back for 8150 gold as you only need it to cast the spell once to store in the field, or better yet just buy a scroll or hire another caster to cast the spell for you. That's much cheaper than a 102000 gold for a 9th level Siberys Dragonshard in a 1 slot Drake-Helm.
Energy Transformation Field allows any caster to have infinite spells. Another good choice for a spell to put into a field is the 9th level spell Absorption (SpC). Put a Maximized Absoprtion in the field, activate it, and you get 10 spell levels which you can use to cast any spell you know. In other words, reach in your Bag of Holding and click the button as a move action to activate the field and get Absorption cast on you. Then draw your special trapped item outside of the Bag of Holding and click it again as another move action to get yourself targeted by 9 Magic Missiles, which converts the Absorption to 9 spell levels you can use to cast any spell you want. It's effectively infinite spells!
Combine free Time Stops with free Absorptions (in two Bags of Holding with Energy Transformation Fields), along with the ability to cast Miracle (such as because you're a Cleric, an Arcane caster with the Luck Domain, or an Arcane Caster with 10 levels of Rainbow Servant), and you can do what amounts to the Team Solar buffing trick with a single character, using infinite Miracles to give yourself every buff of 7th level and below of every class (8th level and below if it's on the Cleric list). And all in a single round!
Single round buffing is one case where the Spelldancer PrC for metamagic reducing (where you dance on your own as much as you want to reduce metamagic) works easier than Incantatrix (where you use Body Outside Body clones to avoid the per day limits), because Time Stop only affects you and not other entities such as your clones.
Fields can be used offensively! If you can cast one near an enemy caster, or trick an enemy caster into moving into a location where Energy Transformation Field has been cast, it's effectively an antimagic field that completely disables all their spells and activatable magic items. Smart casters may have Contingency set up along the lines of "if I'm about to be affected by an antimagic field". Optimized casters also have ways to still cast within an antimagic field, such as the spell Invoke Magic or having the feat Initiate of Mystra. However, there are no ways to cast inside Energy Transformation Field, and few think to include that spell in their Contingencies. Since Energy Transformation Field is a spread instead of an emanation like the Antimagic Field spell, you can't hide behind a tower shield to give yourself cover from it, and it won't activate tricks like a Shink Item'ed giant conical hat that you use to protect yourself by automatically generating cover in case you step into an antimagic field.
