A couple gems, and a couple head-scratchers here.
5. Knock: Is it just me, or is there a correlation between the brevity of the spell description and the usefulness of the spell? Door, chest, gate, box, trap door, book, or anything else that's made to open, Knock will open it, whether it's locked, stuck, barred, magically sealed, or whatever. It's quicker, easier, and more versatile than a thief's Open Locks ability, but at the cost of being a limited resource, so there's no reason why the two can't peacefully coexist and supplement one another in a party. Hey, it's got a range of 60', too, so you could safely open most trapped doors and containers. (The spell description explicity states that it opens the door, chest, or whatever, rather than merely unlocking it.) How about springing the buckle on the enemy fighter's sword belt or backpack strap? Knock as combat spell? Why not? And between adventures, hangin' around the ol' tower, a magic-user with Knock in his repertoire need never fear the most stubborn pickle jar lid.
6. Levitate: Another spell with a pretty simple description, and naturally one of my favorites. It only facilitates vertical movement, but doesn't prohibit horizontal movement by some other means, such as pushing with one's hands along a ceiling. Thus, presumably it doesn't halt horizontal momentum, so a caster under the effect of Levitate could take a running start, leap, and float an almost unlimited distance horizontally without changing direction (subject only to slowing by air resistance, and any solid objects in his path, of course.) Almost nothing is out of reach to a magic-user with a Levitate spell handy. It's also great for keeping the caster out of melee range in a fight, if the ceiling is high enough (though it may make him a blatantly obvious target for missile fire.) Outdoors, it's the ultimate scouting spell - there's no need for a painstaking climb to the top of a mountain or tall tree to get your bearings - just cast and rise as high as you need. Combine with Invisibility for stealthy reconnaissance of enemy territory. The spell description specifies that the caster may carry a normal amount of weight, possibly including another character. Besides the obvious, "another character" could conceivably include a captive - hoisting somebody up to 1,000 feet and threatening to drop him might be an effective method of interrogation. As if all that isn't awesome enough, it's got a good long duration of 6 turns + caster's level, so even a level 3 magic-user (the minimum for casting 2nd level spells) gets a full 90 minutes of floaty goodness out of it.
7. Locate Object: Let me get this straight: You can throw Sleep, one of the most potent attack spells in the rules, 240'. You can make somebody invisible from 240' away. But when you need to find something which presumably is difficult enough to find that you consider it worthwhile to expend a spell slot to find it, you're limited to 60' + 10' per caster level? That's not completely useless - that range extends 360 degrees around the caster, so you're effectively "searching" a circle 120' + 20' per caster level across. Still, it's an odd design choice, considering that range is a much more critical factor for this sort of thing than it is to the above-mentioned Sleep and Invisibility spells and others. Its duration is a paltry 2 turns, so casting it and then wandering the dungeon waiting for your spider sense to tingle when you're near the desired object isn't a very viable strategy either.
It would be great for finding the proverbial needle in a haystack - say, a small item in a cluttered hall - but not much use for finding a staircase in a megadungeon, unless you're already pretty close to a staircase. (Curiously, finding a staircase is an example given in the spell description.)
Expanding the range to yards outdoors (assuming you don't interpret the spell's range as an area of effect instead, which it kind of is,) Locate Object might be useful in town excursions to find desired goods in a crowded market or to track down the pickpocket who's just made off with your enchanted dagger. In the wilderness, you might use it to try to find food, water, or some other resource, although the limited range relative to the vastness of the wilderness itself and its relatively short duration make that an uncertain prospect at best.
At any rate, Locate Object lets you "search" for either a specific object, in which case you must know exactly what it looks like, or the nearest example of a general type of object (i.e. "dagger" or "stairase.") The spell description pretty strongly implies that the item or type of item must be chosen when the spell is cast, and cannot be changed during the duration.
8. Mirror Image: The visual effect sounds pretty cool, but I can't think of a lot of "outside the box" uses for this spell. Essentially, it gives the caster a few (1d4) decoys which allow him to soak up a few attacks without being harmed. What else is it good for? I'm not sure. Because the mirror images move exactly in synch with the caster, it's likely that intelligent creatures would know right away that the "extras" are not real and wouldn't be fooled into thinking they're facing greater numbers than they really are. The images wouldn't even make good seat-fillers at the annual Mages' Guild Awards. They might fool animals and low-intelligence creatures, though. Maybe you could keep a pack of wolves at bay by bolstering the party's numbers with a few mirror images.
The spell description doesn't explicitly say so, but the only range given for the spell is 0 (caster only) so I'm assuming the mirror images cluster fairly closely around the caster, and you can't project them any real distance.