Consumptive Field (Clr4, Libris Mortis), and Greater Consumptive Field (Clr7) grant 1d8 temp hp per creature killed by the field. Walk around in a chicken farm with one of these and you can get thousands of temp hp.
There are some polymorph spells specific to a single creature-type which, unlike general-purpose polymorphing, do increase your hp. The ones that grant a lot are:
- Dragonshape (Sor/Wiz9, PHB2): 150 temp hp.
- Aspect of the Platinum Dragon (Sor7, Dragon Magic): 110 temp hp.
- Aspect of the Chromatic Dragon (Sor8, Dragon Magic): 100 temp hp.
- Lesser Dragonshape (Sor/Wiz6, PHB2): 60 temp hp.
- Unyielding Form of Inevitable Death (Sor/Wiz8, Complete Mage): 50 temp hp.
- Vigor(1st level Psionic Power) grants 5 + 5 per every extra powerpoint spent temporary hitpoints.
If you're already high character-level, then anything that grants bonuses to Constitution has a large effect on your hp. (As long as it's actual "bonuses", not just replacing your physical ability scores via polymorph.) The largest single such bonus I know of is Giant Size (WuJen7, Complete Arcane) which grants a +12 size bonus to Con, which translates to +120 max hp if you have 20HD. Or you can stack several smaller bonuses of different types.
Righteous Fury (Pal3, Spell Compendium) grants 50 temp hp.
Fortunate Fate (Clr7, Spell Compendium) is a contingent Heal, which is sort of like having casterlevel*10 temp hp.