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.