Can you activate a land's mana ability while a spell is on the stack?

Mana abilities are a special category of activated abilities that produce mana as part of their effect. Unlike normal activated abilities, mana abilities do not use the stack and do not require a player to have priority in order to be activated. This is explicitly covered in CR 605.3a.

Because mana abilities resolve immediately without going on the stack, they can be activated at virtually any time a player needs mana — most commonly in the middle of casting a spell or activating another ability, when you need to pay a mana cost. This is the core mechanic that allows tapping lands to pay for spells.

A mana ability is defined in CR 605.1 as an activated ability that (a) is not a loyalty ability, (b) doesn't require a target, and (c) could add mana to a player's mana pool when it resolves. Basic land tap abilities like Forest's '{T}: Add {G}' fit this definition perfectly.

Concrete example: You cast Counterspell (costing {U}{U}). While Counterspell is on the stack waiting to resolve, you realize you only have one blue mana floating. You can tap two Islands using their mana abilities right then — without needing priority — to generate the {U}{U} needed to pay the cost.

Note that some abilities that produce mana DO use the stack if they require a target (CR 605.1b) or are triggered mana abilities (CR 605.3b). Those rarer cases work differently, but standard land tap abilities are always free to activate when mana is needed.