There are lots of good low level buff spells, like Chant and Bless.
Another good buff spell is Blood Wind, from SpC. Level 1 and only a swift action, it would let your big demon use his full natural attack routine from range for 1 round!
Close Wounds can help keep allies alive rather well, especially if it's a group of low level clerics continually saving each other's asses.
Benign transposition (SpC) can be used to help maneuver the main bad guys into better positions, using the minions to move to said location and then swapping positions.
Nerveskitter (SpC) is a level 1 immediate action spell cast when initiative is rolled, and adding +5 to the target's roll. Useful to let the important NPCs go first.
Seeking Ray (PH2) is probably the most useful ranged damage spell a minion could use, although they're best for non-damaging spells.
Persistent Blade (SpC) is level 1 and does pitiful damage, but does leave a floating weapon locked on the target (like spiritual weapon), and the main benefit: that target is considered flanked by anyone attacking him in melee. Blackguards get SA, right?
Mass Snake's Swiftness (SpC) is only level 2 for druids, and gives all targets an immediate free melee attack.
Mark of Doom (PH2) is a pretty nasty curse-like effect, Cleric level 3. It offers no save or SR (!), has good range, and for round/level, whenever the victim attacks or does other aggressive actions (see spell description), he takes 1d6 damage! That can add up really fast for say...a TWF.
Wings of Cover (Races of Dragon) is a level 2 immediate action Sorc-only spell that..completely negates one attack or block line of effect long enough to ruin a targeted spell. If the caster is dragon-blooded, he can use it to protect an adjacent ally instead of just himself. Very useful, although it's a pretty cheesy/broken spell.
Haste is always a useful group buff. Similarly, Slow could be worth it if even just one PC fails the save. Girallon's Blessing (SpC, Clr/Drd/Wiz/Sorc 3) is another great buff spell for a single target, and at 10 min/level, could evne be cast out of battle, if the NPCs are expecting company.