I have only a basic knowledge of most of these mods, and as such some information may not be totally accurate or relevant. I encourage anyone with a good understanding of the mod to give their own input on the matter.
A collection of mods to the game that affect the strength of certain nations, spells or units.
A balance mod made by a member of the /domg/ community - 'Kate Miccuci', and later taken over by NewCrone, it makes changes to several spells - most notably Burden of Time and summoning spells - making stronger spells a bit weaker, and weaker spells a bit stronger, balances the costs of several nations units, gives certain nations additional magical paths and general makes weaker units, spells and items more viable.
Another big balance mod, made by a member of the Dom4Mods community - 'Johannes', it places a focus on equalizing the power levels of all nations towards the stronger, while putting some restrictions on some of the strong Army Every Turn nation (eg; Mictlan, Niefel/Jotunheim, etc). It also makes minor changes to some items, spell costs, making less useful spells cheaper and making big global spells more expensive or moving them higher into the research tree.
An underwater focused balance mod, made by a member of the dom4mods/desura communities - 'Ohlmann', this mod makes changes to underwater nations, gives certain spells the ability to be cast underwater, and generally makes underwater nations a lot more viable in comparison to land nations. Most notably, it gives undead and ethereal summons (poor)amphibious status, lets underwater nations get onto land easier and lets land nations get into the water easier, gives more underwater pretender options, and in general makes underwater units a bit stronger.