You know how a wizard gets two more spells added to their spellbook every time they level up? If you're a specialist wizard, one of those spells must be from your specialty school.

With reach weapons, with regards to cover or concealment, you are treated as wielding a ranged weapon, which means you can gain the benefits of Improved Precise Shot with a spiked chain or glaive or whatever. It also means a rogue using a spiked chain (or any actual ranged weapon, for that matter) can sneak attack enemies with concealment (they still need to be able to see clearly).

Armor spikes let you deal their damage on top of your unarmed damage when you grapple somebody.

It's not a "rule", but a bard with 18 Int can have max ranks in every knowledge skill.

One of the options for weapon focus/specialization is "grapple"

Craft Magic Arms & Armor/Wondrous Item let you fix broken items at 1/2 their normal creation costs (1/4 their market costs); sunder ain't so bad after all

Improved Turning doesn't just add +1 to your turning damage, it also adds to your level as per table 8-9

Spirited charge triples your damage with a lance; this means any flat bonus as well, naturally, so power attack and even smite evil for some truly prodigious damage

A halfling under the reduce person spell is tiny and can therefor ride an eagle (such as a druid's animal companion, or a 1st level summon natures ally); giving a party some access to flight as early as 1st level

The snare spell requires a search check to notice it (not a spot check), with a DC high enough to require trap finding, and allows no save and no SR. Breaking free requires a full-round action, meaning this spell is a guaranteed way to inhibit even the most powerful enemies, for at least a round (unless they can teleport).

Tripping a flying creatures causes it to stall.

Most flying creatures have a minimum forward speed; a net slows your speed to half, therefor using a net on a flying creature may take away it's ability to fly at all, or maybe just force it to fly in a straight line until landing.

Protection from X spells prevent summoned creatures from touching you, meaning many can't attack you at all, this includes summon natures ally.

Due to uncanny dodge, rogues and barbarians retain their dex bonus to AC against surprise attacks. This means they retain any dodge bonuses, such as from trap sense, total defense, or combat expertise. So, a rogue or barbarian, with a shield, taking a total defense action, can walk down a trapped hallway without worrying about trap attacks (pits and magic ones are still deadly, of course).

A monk retains their unarmed damage while using a gauntlet, though they can't flurry with it.

Improved cover grants a +10 bonus to hide checks, so peeking around a corner is actually a thing in the rules.

Deafened enemies suffer a -4 penalty to intiative, giving thunderstones a decent surprise round use in parties containing rogues.

Feinting bypasses dodge bonuses, so if you can't hit that damn swashbuckler, try feinting.

You can dismount a mounted character with a trip check (though they can use their ride check to oppose). Still trying to figure out how this ties into jousting...

Wanna take out a big monster with the swallow whole ability? Use resist energy (acid), stoneskin, and maybe shield other, get swallowed, and start attacking at max capacity, but don't actually make a hole to escape with. (technically not a "rule" either)

Implosion is not a death spell, so it bypasses death ward.

Restoration heals fatigue, allowing one to continue sprinting or traveling long distance (good for mounts).

I'll stop there. I'm always finding new things in core 3.5.