<cranky old man voice>
Let's get one thing straight up front. A martini is gin with a splash of Vermouth. Regardless of what James Bond and 10 million drunken college kids will tell you, there is no such thing as a vodka martini.
You want a good martini, use good gin. I like Bombay Sapphire or Quintessential myself. If you want variety, use a twist or even a lime instead of an olive. But don't go messing with the basics - its gin and Vermouth. Period.
</voice>
That out of the way, you can mix up just about anything you want and as long as you serve it in a martini glass, you can get away with calling it a martini. Here's my favorite:
Ingredients for Lemon Drop Martini
2 ounces lemon flavored vodka (for example, Absolut Citron)
1 teaspoon sugar
lemon wedges
lemon twist and sugar to garnish
Instructions
1. Place the lemon vodka, sugar and one lemon wedge in a cocktail shaker with crushed ice.
2. Shake until well blended
3. Serve in a martini glass that has the rim coated in sugar
4. Garnish your lemon drop martini with a lemon twist.