Off-hand weapon damage bonuses

3.5 Rules
If you wield a weapon in your off-hand, you do not get to apply your full strength bonus to damage from your off-hand attack.  Thus, if you have a strength of 18 and you fight with a long sword and a dagger, you can add +4 for your strength bonus to attacks made with the long sword, but only +2 for your strength bonus to attacks made with the dagger in your off-hand.
You can find this information on page 134 of the Player's Handbook.

Benign Transposition - a favorite 2nd level spell

3.5 Edition Rules:

One of the spells we have had the most fun with is a 2nd Level spell from the Spell Compendium called Benign Transposition.  My Rogue/Wizard has it and usually has 2 memorized each day.  She has used it to take the place of another party member on the verge of death and she has used it to put herself inside a trap that she was small enough to get out of but that the other party member could not escape.  She used it to replace the party monk in the mouth of the T-Rex with the heavily armored party dwarf, who was then enlarged by a cleric with the Enlarge Person spell which trapped the dwarf in the maws of the T-Rex, but also prevented the T-Rex from spitting out the dwarf or swallowing the dwarf or attacking anyone else.  She has also moved into flanking position behind powerful enemies that ignored her because of her small size, then used the spell to replace herself with the party half-ogre who is definitely more threatening to most enemies.

If you take a moment to learn this spell, I suspect you too can find many creative uses for it.

Fun Stuff - A Pentagram on the Ceiling

As DM, I once had the party enter a room with a pentagram painted on the floor and a succubus contained within it.  The party, knowing the rules of pentagrams, entered the room assuming the succubus could not attack them as long as they did not cross the boundary of the circle around the pentagram.  Despite their careful movement, it attacked them and the party fought and killed the succubus.  Only after the battle, when the party was wondering why the pentagram did not contain the succubus, did they notice the larger pentagram painted on the ceiling above; which was the true container for the demon.

Flat footed and initiative order

3.5 Edition Rules:

Catching your opponent flat-footed is an advantage of high initiatives that is easy to overlook.  From page 137 of the Player's Handbook:  “At the start of a battle, before you have had a chance to act (specifically before your first regular turn in the initiative order), you are flat-footed.”

This gives characters the same bonus they would get when surprising an enemy.  This is especially valuable to rogues because it can allow them to apply sneak-attack damage to their attacks.

Dealing damage by a Halfling rogue

3.5 Edition Rules:

My Halfling rogue long envied the amount of damage her fellow party members could deliver to opponents.  Her small hand crossbow, while convenient and light-weight, delivers only a d3 of damage sans modifiers.  Though it gives her range and keeps her out of the thick of the battle, she rarely catches the enemy flat-footed and thus cannot apply her sneak-attack damage.  Recently, that all turned around when she acquired a ring of invisibility.  Being invisible, her enemies are denied their dexterity bonus to AC, thus making them vulnerable to sneak attack damage.  She also has taken the crossbow sniper feat which allows her to perform sneak attacks from 60' instead of just 30'. 

Bottom line:  being invisible is very good for ranged sneak-attacks from rogues.

Quick Draw benefits for throwing weapons

3.5 Edition Rules:

If your would like to play a character that throws daggers or other weapons, then you should have your character take the Quick Draw feat.  Not only does Quick Draw allow you to draw a weapon as a free action, it provides a secondary benefit of allowing a character to throw one weapon per his number of attacks instead of being limited to throwing one weapon per round.

Apply your sneak attack damage to all your attacks

3.5 Edition Rules:

We applied our sneak attack damage to a rogues first attack for a long time.   We finally discovered that we were playing it wrong, and immediately our rogues became must more dangerous in combat.   The correct method is to apply sneak attack damage to all rogue attacks that qualify for a sneak attack.   This means that an 8th level rogue, with a Base Attack Bonus of +6/+1 and wielding two weapons and thus getting 3 attacks (2 from his primary hand and 1 from his off hand) gets to apply sneak attack damage to all 3 attacks.   If he hits with a long sword in his primary hand twice and a short sword in his off-hand once, the damage would be (d8+ 4d6) + (d8+4d6) + (d6+4d6). That is potentially a lot of damage.
Source: http://www.wizards.com/default.asp?x=dnd/rg/20040217a

Overcoming Damage Reduction with a Sneak Attack

3.5 Edition Rules:

Many situations were not clearly addressed in the 3.5 edition of books.  One case is how to treat sneak attack damage when attacking an opponent with Damage Reduction (DR).  Fortunately for us, this case is specifically addressed on the Wizards site in one of the excellent "All about..." series.  In "All About Sneak Attacks (Part Three)", Skip Williams tells us that the damage from a sneak attack does count toward the amount needed to overcome DR.  Therefore, if your rogue deals 4 points of weapon damage against a creature with DR5, and includes 7 more points of damage from the sneak attack, the total damage of 11HP is enough to overcome the DR5 and the creature will take (11-5)=6 points of damage from the attack.

Source:  http://www.wizards.com/default.asp?x=dnd/rg/20040302a