This came up during a commander game this evening. Someone targeted Ghave, Guru of Spores with a damage spell doing just enough to kill him. Can I use his first activated ability to take a counter off another creature and create a Saproling, then after that resolves sacrifice that Saproling to put a counter on Ghave, all before the spell on the stack resolves? It was decided this didn't work, but I'm not sure.
Yes, you can do this.
After Ghave has been targeted with a damage spell, the damage spell goes on the stack. You then respond to that spell by using Ghave's first activated ability, which also goes on the stack.
At this point (as long as nobody else plays in response), each player has passed priority, triggering rule 608:
608.1. Each time all players pass in succession, the spell or ability on top of the stack resolves.
Ghave's activated ability resolves, and a Saproling enters the battlefied. Rule 116 then comes into play:
116.3b The active player receives priority after a spell or ability (other than a mana ability) resolves.
Priority returns to the active player (which could be you or your opponent) before going around to each of the players again. No other spells or abilities on the stack will resolve until each player has passed priority.
When you get priority again, you can use Ghave's second ability to sacrifice the Saproling and place a counter on Ghave to save him.