Most of what I'm about to suggest isn't normal "buffs" but rather good ways to react to bad things. The good news is that they don't require you to plan for what might happen ahead of time.
I like the Close Wounds spell. It's immediate, so you can use it even when it's not your turn, and it can "undo" damage in the sense that if someone dies from a blow you can cast the spell and say, "Nuh uh."
I also like Benediction (from Complete Champion), which basically grants the luck domain to a character for an hour or two. And from the same book, I also like Protection Devotion. At 9th level, it would grant a +4 to AC to every ally near you, and it's also immediate, so you can interrupt someone getting walloped to say, "I activate the Protection Devotion feat, and you're now 4 points tougher to hit."
Blessing of the Righteous from PHB 2 grants an extra d6 of damage to all your allies' weapons. Lastly, from the same book, I've begun to take Alter Fortune as my emergency spell. It costs 200 XP to use, so you pretty much never want to use it, except for that moment when someone just rolled devastatingly badly, and terrible consequences are about to happen. For example, our group couldn't cast Teleport, but we had a sorcerer with a scroll of it. Following the rules for casting a spell of higher level than you can normally cast, she needed to roll anything on a d20 except a one in order to succeed. If she failed, we were staring down a TPK. And of course, she rolled a one. We didn't all die, but in our very low-cash game, the loss of the scroll alone was miserable enough. I would have gladly burned the Alter Fortune spell to grant her a reroll, if I had known about it at the time.