Suppose a spellcaster wishes to use a heavy shield, or even a heavier shield like a tower shield. Divine, so arcane spell failure doesn’t apply, but they are using a weapon. Since heavy (and heavier) shields use the hand, you cannot simply pass the weapon into your other hand like you could with a light shield (and certainly cannot perform somatic gestures with the shield hand, as you could with a buckler).
Complete Mage has the Somatic Weaponry feat that gets around this, but a feat seems like an awful lot to expend for what amounts to a +1 to +3 additional shield bonus to AC. Particularly considering that characters with two-handed weapons, already better off, don’t have to do anything at all; they can just let go of the weapon and cast the spell.
Anything that requires a non-free action in combat is useless, so dropping or sheathing the weapon is not a good answer (unless you can somehow cheaply make these approaches free actions, but I think that is impossible). But long-term buff spells (especially those that are wand-able), magic items, mundane item alterations, etc. are all great. Particularly efficient dips might be ok, especially if it’s just one level that is also doing other things for you (double particularly if it progresses spellcasting). Feats that allow this and also do other things are good, too.
Please cite sources, and in particular note any materials coming from third-party sources. Material must have been published for D&D 3.5e and/or Pathfinder to be considered.
Best Answer
Glove of the master strategist: (3.600 gp, Ghostwalk pg 71). As a free action, store shield inside the glove or make it appear in your hand. It depends on the table if a retrieved shield is considered donned or simply held when it gets out of the glove. As an overpriced alternative, Glove of storing has the same effect.
Arms of the Naga: (56.000 gp, Savage Species pg 55). Two extra arms that can carry your shield for you, but they are expensive.
Girallon's blessing: Spell from the spell compendium that gets you two extra arms. It's a 3rd level spell, so it is wand-able if you do not mind spending 11250 gp on a wand. Decent duration too (10 minutes/level).
I would also mention Shielded mage feat, from Pathfinder. It's a poor candidate because it has Shield focus as prerequisite, costing two feats for the same effect as Somatic Weaponry plus a measly +1AC. The only redeeming point is that you can have it as early as level at level 1 if you don't mind a 1 level dip in fighter (which you need anyway if you ever want to use a tower shield).