Giants: Loud and Deep voice. (Banging on the table is also a good effect - move their dice
)Tiny or smaller creatures: High Pitched.
Intelligent monsters found in isolated places: Have them talk a LOT. It has probably been ages since they last spoke with someone. Perhaps they talk really fast, never giving you a chance to get anythign in edgewise. Or just go on an on: "You would like to know my name? Well I would be glad to tell you. And tell you now, without any further delay. Becaus my name is very important to me. I am named after my Great-Great Grandfather...."
Evil Creatures: Lots of things you can do here. Evil Laugh is just the basics. You can go for the nihilst, "I don't care about anything" route. Or you can go for the always angry route. Or the always Insulting route.
Powerfull Good Creatures (Angels, Archons, etc.) : Harder to do. You do now cant to be to annoying. One possibility is the Parent speaking to child approach.
Creatures with an intelligence of 1-2: They may not speak, but do not forget to growl/whimper. And do not say "the tiger charges and growls" Instead say "the tiger charges forth with a mighty RAAAAAAUUUG!!!!". And a good low volume, high pitched "hhhhmmmm" can indicate wounded creatures.
Undead: They rot. That includes their toungue. It can be very annoying to face the vampire/lich who says "ge ou of my air!" Because he can not make the T or and L sound. :D Just try not to move your toungue when you speak.
For Ghosts or similar creatures you can have themnot realize they are dead.
Fey: Has many stereotypes. Try your hand at rhyme, riddles, or just being jaded and superior.
Outsider: They are from another place right? Consider them to be tourists, very curious, not understanding everything around them, goin oooh, and ahh, or being snobbish. "We would never let a dragon loose back at the Happy Hunting Grounds. Where are your toilets? IN THE WOODS?????? Well I never."
Plants: Heavily object to fire, vegetarians/herbivores, and axes. May speak very slowly. "HHHeeellllllooo. III aaammm ccccaaalllllleeeddd tttrrreeaa...."
Intelligent Vermin: Touchy: "You think your better than me? Just because I am a cochroach and you are a human? Well I got news for you buddy..."
And what about the ever-overlooked antagonist: the BBEG's sidekick? He might be shy and withdrawn, or he might try to mimic the BBEG's actions and mannerism right down to his speach patterns. The sidekick doesn't just have to be for comic relief, either. A sidekick can be a powerful, unique character in his or her own right.
Another way of increasing the players 'feel' for the NPCs is through use of body language.
If you are trying to depict a stereotypical:
Giant - frown and raise your shoulders, try and lift yourself over the players, even go so far as to stand.
Rogues - as rat-like, narrowing your eyes, scrunching up your nose, hunch and curling up your lip to match his voice.
A Paladin/Knight - straight back, lifted chin and intense look. If they have a beard/mustache.
Wizard - thoughtful expression maybe coupled with (if they possess one) habit of stroking their beards while pondering a question and adjusting their glasses.
Cleric - think paladin with frequent looks to the heavens and unconscious play with their holy symbol.
Nobility - draw yourself up and look down your nose, maybe they pick up items with their 'pinkies' aloft, maybe act out them snorting snuff, perhaps make large gestures to match the pompous speech.
Pesants - repectful, bow, wide-eyed,
Disgusting/ Unintelligent - similiar to giant, scatch your head, pick your ear, wipe your nose, leave your mouth open
When creating NPC that will meet the players, try and think of voices and mannerisms that match what you want the players to think about the NPC.
For example a humorous NPC will usually be liked, where a angry aggressive NPC will be disliked.
Making the NPC voices and acting out their body language is great fun and really gets the players 'believing' in the creature. It is a great way to encourage roleplay in your players too.
;)