List of martini variations

Many variations exist on the standard martini.

A bottle of Martini Extra Dry.

Variations on the proportions

A "dry martini" uses less dry vermouth than normal, perhaps a dash or lace of the glass. Similarly, a "wet martini" refers to a martini that uses a greater amount of Vermouth.
A "50-50 martini" uses equal parts gin and dry vermouth.
A "perfect Martini" is technically one made with a mixture of equal parts dry and sweet vermouth, although in many bars the term is misused as a qualitative one.
An "upside down martini" or "reverse martini" uses more vermouth than gin.

Variations on the gin or vermouth

A Vesper is a variation also favoured by James Bond, which is made with three measures of gin (Gordon's was Bond's preference), one measure of vodka (grain vodka is preferred), and half a measure of Kina Lillet aperitif, shaken until ice-cold, and with a large, thin slice of lemon peel for garnish.
A Vodka Martini (aka. Kangaroo) substitutes vodka for gin, and often uses lemon rind as the garnish. This is the most common variation. It was made famous by the James Bond movies as James Bond's favourite beverage. He is known for requesting it "shaken, not stirred"

Variations on the garnish

A Dirty Martini has some of the brine (at least a teaspoon) from the olive jar added.
A Gibson is a standard dry martini garnished with cocktail onions instead of olives.

Variations on serving

A martini on the rocks is served on ice, in a rocks glass, instead of being strained into a cocktail glass.

Total variations

Sometimes the term "martini" is used to refer to other mostly-hard-liquor cocktails such as Manhattan (cocktail), Cosmopolitan (cocktail), and ad hoc or local concoctions whose only commonality with the drink is the cocktail glass in which they are served. Chefs with a more whimsical bent are even producing dessert "martinis" which are not a drink at all, but are merely served in martini glasses.

Another popular form is the espresso martini, popular in restaurants as a dessert. Many variations exist but most involve shaking an espresso shot with the ingredients and served in a chilled martini glass. By shaking a fresh espresso shot it creates a hard layer of crema which is garnished with three coffee beans in the centre.