Here is one I used for an old 3rd Ed campaign. I threw it together, feel free to modify whatever you like.
Class Level Base Attack Fort Ref Will Special
1 0 0 2 0 Death Attack, SA 1d6, Poison Use, Improved Unarmed strike 1d6
2 1 0 3 0 Poison Craftsman, Evasion
3 2 1 3 1 SA 2d6, Painful strike
4 3 1 4 1 Uncanny Dodge
5 3 1 4 1 SA 3d6 , Poison Reaper
6 4 2 5 2 Life is cheap, Uncanny Dodge (can't be flanked)
7 5 2 5 2 SA 4d6, Silent strike
8 6/1 2 6 2 Blinding Strike
9 6/1 3 6 3 SA 5d6, Trap craftsman
10 7/2 3 7 3 Crippling Strike
11 8/3 3 7 3 SA 6d6, Uncanny Dodge (+1 vs. traps)
12 9/4 8 4 Life is worthless
13 9/4 4 8 4 SA 7d6 Improved Evasion
14 10/5 4 9 4 Uncanny Dodge (+2 vs. traps), Agonizing Strike
15 11/6/1 5 9 5 SA 8d6
16 12/7/2 5 10 5 Opportunist
17 12/7/2 5 10 5 SA 9d6, Uncanny Dodge (+3 vs. traps)
18 13/8/3 6 11 6 Wounding strike
19 14/9/4 6 11 6 SA 10d6
20 15/10/5 6 12 6 Uncanny Dodge (+4 vs. traps)
Alignment: Any non-lawful
Hit die: d6
Class Skills: Balance, Bluff, Climb, Concentration, Craft (poison), Craft (Traps), Diplomacy, Disable Device, Disguise, Escape Artist, Forgery, Gather Information, Hide, Innuendo, Intimidate, Intuit Direction, Jump, Listen, Move Silently, Open Locks, Read Lips, Search, Sense Motive, Spot, Swim, Tumble, Use Rope
Skill points at 1st Level: (6 + Int modifier) x4
Skill points at each Additional Level: 6 + Int modifier
Class Features:
Weapon and Armor Proficiency: An assassins training includes proficiency with the following weapons: crossbow (hand, light, and heavy), dagger, dart, rapier, sap, shortbow (including composite), shortsword, club, quarterstaff, Long knife, and garrote. Assassins are proficient with light armor only.
Sneak Attack: The assassin may sneak attack exactly as a rogue does. Sneak attack damage does not stack with rogue levels but does stack with any prestige class, which grants this ability.
Poison Use: Assassins are trained with poisons and never risk accidentally poisoning themselves when applying poison.
Improved Unarmed Strike 1d6: Assassins are efficient killers and are never unarmed. All assassins may make unarmed attacks that inflict 1d6 damage. The assassin is considered armed and may deal subdual damage if desired.
Death Attack: If the assassin studies his victim for 3 rounds and then makes a sneak attack with a melee weapon that successfully deals damage, the sneak attack has the additional effect of possibly either paralyzing or killing the victim (assassin’s choice). While studying the victim, the assassin may take other actions so long as his attention stays focused on the victim and the victim does not detect the assassin or recognize the assassin as an enemy. If the victim of such an attack fails a Fort save (DC 10+ assassin class level+ Intelligence modifier,+1 for every level the assassin is above the victim, -1 for every level the assassin is below the victim) against the attack, he dies or is paralyzed for 1d6 rounds plus one round per level of the assassin. If the saving throw succeeds the attack is just a normal sneak attack. Special: If the victim is subject to a coup de grace the assassin need not spend any time studying to make the attack and the victim must make the save at DC 15 plus all applicable mods.
Poison Craftsman: The assassin gains a +4 bonus to the Craft (poisons) skill.
Evasion: This works exactly as the rogue ability.
Painful Strike: You are able to strike where it counts. You may target sensitive areas causing tremendous pain. As a full attack action, you may make one attack at your best attack bonus. If you hit your opponent, he must make Fortitude save (DC 10 + assassin level) or be partially overcome with pain. Your foe receives a –2 circumstance penalty to all attacks, saves, and damage rolls until your next action. This ability only functions against opponents who are subject to critical hits. This ability replaces sneak attack damage if that would apply.
Uncanny Dodge: This works exactly like the rogue ability.
Poison Reaper: You can extract one dose of poison from a dead creature that uses venom. The creature may not have been dead for more than 24 hours and it takes one hour of work to extract the poison. The extraction requires a craft (poisons) check (DC 20). If successful you gain one dose of injury poison from the creature’s venom. This poison functions exactly like the source creature’s venom when applied to a weapon with the save DC decreased by 1d4 due to exposure to air.
Life is Cheap: Due to their ability to recognize an opening when it presents itself, an assassin of 6th level or higher may make a coup de grace as a standard action instead of a full round action.
Silent Strike: When attacking from hiding with a ranged weapon, the assassin’s opponents must make a Spot check in order to determine his location. For each attack the assassin makes after the first, his opponents gain a +2 competence bonus to their Spot checks. This bonus accumulates for a particular foe until he discovers the assassin’s location.
Blinding Strike: Your mastery of underhanded yet efficient tactics allows you to temporarily blind your foe. You may use the full attack action to make an attack that blinds your foe. Make a single attack using your best base attack bonus. If you hit, your opponent must make Fortitude save ( DC 10+ assassin level) or be blinded for 1d4 rounds. This attack only works once per battle against a given foe. Only creatures that use visual organs to see are vulnerable to this attack. This ability will only function against opponents who are subject to critical hits. This ability replaces sneak attack damage if that would apply.
Trap Craftsman: The assassin gains a +4 bonus to the Craft ( Traps) skill.
Crippling Strike: This works exactly like the rogue ability.
Life is Worthless: At this level the assassin may make coup de grace attacks as a stand action without provoking attacks of opportunity.
Improved Evasion: this works exactly like the rogue ability.
Agonizing Strike: When making a sneak attack you may choose to stun your opponent rather than inflict damage. Using a full attack action to execute a sneak attack you may opt to stun rather than inflict sneak attack damage. Your opponent takes normal damage from the attack and instead must make a Fortitude save ( DC 10+ assassin level) or be stunned for a number of rounds equal to half your assassin level.
Opportunist: this works exactly like the rogue ability.
Wounding Strike: Your sneak attacks are devastating. A successful sneak attack deals 1d2 points of Dexterity damage to the target. This effect stacks with Crippling Strike.