Armor class
Armor class (hereafter referred as AC) is the simplest and most common way of not getting hit. Apart from not playing D&D, it is also the most cost-efficient way of doing that as there are dozens of possibilities to acquire AC and getting cast amounts of it tends to be easier than trying to get your attack bonus to match optimized AC.
Some spells have been neglected on purpose. For example, I am aware that Greater Visage of the Deity gives you +1 to your natural armor, but it hardly is the main effect of the spell and no-one actually uses it to increase his AC. You know what I mean, right?
There are four different AC scores, normal, touch, incorporeal touch and flat-footed. You want to have high scores in all of them, since you never know when someone's using Wraithstrike and might actually hit you. Foresight takes away the chance of being flat-footed, so if you can have that on most of the time, just make sure your touch AC skyrockets with stuff like Scintillating scales. It might also be worthwhile not to depend on any single spell or source of AC.
Armor bonus:Player's Handbook:-Mage armor, +4 bonus, 1h/CL, Wiz/Sorc 1, Force 1, Spell 1
-Magic Vestment, +1/4 Cls (max +5), 1h/lvl, Cleric 3, Strength 3, War 3. Has to be applied to an existing item.
Spell Compendium:-Greater Mage armor, +6 bonus, 1h/CL, Wiz/Sorc 3
Book of Exalted Deeds:-Luminous armor, +5 bonus (-4 to enemy attacks), 1h/CL, Sanctified 2
-Greater luminous armor, +8 bonus (-4 to enemy attacks), 1h/CL, Sanctified 4
Sandstorm:-Ironhorn extract, 25gp, +1 bonus for one week
Dungeon Master's Guide:-Monk's belt, 13k, wisdom modifier +1 to AC while unarmored. Too awesome not to be mentioned, by far my favourite AC trick with Druids and Clerics, who max wisdom like no-one's business and sometimes even higher than that.
Shield bonus:Player's Handbook:-Shield, +4 bonus, 1min/CL, Wiz/Sorc 1
-Magic Vestment, +1/4 Cls (max +5), 1h/CL, Cleric 3, Strength 3, War 3. Has to be applied to an existing item.
-Mithril buckler, +1 bonus. No arcane spell failure, 1015gp.
Dungeon Master's Guide II/Player's Handbook:-Feycrafted darkwood buckler, +1. No arcane spell failure or armor check penalty, 715gp.
Natural armor:Player's Handbook:-Barkskin, +2 bonus and +1 per 3 Cls (max +5 at level 12), 10min/CL, Drd 2, Rgr 2, Plant 2
-Righteous might, +2 bonus, 1rnd/CL, Cleric 5, Strength 5
-Various polymorph-based spells are dealt with in Change your form-section.
Spell Compendium:-Spider skin, +1 enhancement (+1 every for every two CLs after six, max +5), 10min/CL, Drd 3, Sorc/Wizard 3
-Dragonskin, +1 enhancement/ 2 CLs (max+5), 10min/CL, Sorc/Wiz 3. Also gives energy resistance.
-Bite of the X, a variable amount of enhancement, 1rnd/CL, Druid x, Sorc/Wiz x. A buttload of other benefits as well.
Dexterity:Player's Handbook:-Cat's grace, +4 enhancement bonus, 1min/CL, Drd 2, Rgr 2, Brd 2, Wiz/Sorc 2, Celerity 2, Halfling 2,
-Various polymorph-based spells are dealt with in Change your form-section.
Spell Compendium:-Divine agility, +10 enhancement bonus, 1rnd/CL, Cleric 5
Deflection bonus:Player's Handbook:-Shield of Faith, +2 bonus (+1 per 6 Cls, max +5 at level 18), 1min/CL, Clr 1
Sandstorm:-Halo of Sand, +1 bonus (+1/3 CLs, max +4), 10min/level, Drd 2, Rgr 2
Spell Compendium:-Scintillating scales, changes natural armor to deflection bonus, 10min/CL, Wiz/Sorc 2
-Armor of Darkness, +4 bonus (+1/3 CLs above 7, max +8), 10min/CL, Darkness 4
-Miscellaneous:Player's Handbook:-Foresight, +2 insight bonus, 10min/CL, Drd 9, Wiz/Sorc 9, Time 8. Also makes you immune to surprise, so no more being flat-footed when combat starts.
-Haste, +1 dodge bonus, 1rnd/CL, Brd 3, Sorc/Wiz 3, Celerity 4. +1 bonus to reflex saves, increases speed and gives an additional attack each round.
Player's Handbook II:-Visions of the future, +2 dodge, 1h/lvl, Clr 8. Can be discharged for 1/2 CL bonus until your next turn, also gives +2 sacred to saves.
Book of Exalted Deeds:-Exalted rainment, +1 sacred bonus per 5 CLs, 1min/CL, Sanctified 6. Also gives DR, SR and reduces ability damage.
Complete Champion:-Interfaith blessing, +1 unnamed for worshippers of Ehlonna, Obad-Hai or nature, +2 for those of Fharlaghn, Moradin and Pelor, 1min/CL, Drd 2, Clr 2. 20' radius.
Spell Compendium:-Shield of Warding, +1 sacred bonus per 5 Cls, 1min/CL, Clr 3, Pal 2
-Holy Star, +6 circumstance, 3 rounds, Cleric 7
-Silver beard, +2 sacred, 1min/CL, Pal 1
-Sacred haven, +2 sacred, 1min/CL, Pal 4. 30' radius, targets retain dexterity bonus to AC even while flat-footed.
-Divine protection, +1 morale, 1min/CL, Clr 2, Pal 2. 20' radius and also gives +1 morale bonus to saving throws.
-Recitation, +2 luck (+3 if allies worship same deity), 1rnd/lvl, Clr 4, Purification 3. 60' radius.
Dungeon Master's Guide:-Greater magic weapon+Defending weapon, maximum +4 at level 20. Stacks with everything else.
-Change your formPlayer's Handbook:-Alter self. There is an entire thread dedicated to finding good ways of using it. Troglodyte is quite popular a form and for outsiders, Dwarven Ancertor is damn near perfect.
Here is a link to the Alter self-thread.-Polymorph. Not so surprisingly, Polymorph and it's bigger brothers have a similar thread.
Linky.Thanks for PhaedrusXY and MegaPlex for their Alter self and Polymorph threads, respectively. Those should give good insight to help beginners and even the most experienced players in picking the best forms.
Random AC bonuses for casters:Complete Champion:-Law devotion, 3-7 sacred/profane for a turn attempt.
+5 Lucky* Parrying Spellstrike Warning Shuriken. It only gives +1 insight to AC, but it is cheaper than an Ioun Stone by a margin of 993.4 GP. And, lots of other benefits. Like +5 insight to init, +5 unnamed all saves, +1 insight to saves (total +6)... The Lucky can be replaced with another +1 property if you aren't a ray wizard. General purpose item that still helps. Catch is you have to hold it. If this is a problem because both hands are needed elsewhere, Spare Hand works but removes the cost efficient for pure AC factor.