You can buy lemon and lime curd, supply stores have custard and creme fillings, pudding, canned creme pie fillings.
Theresa 
Some ideas:
fresh berries macerated with a little liquour (drain them before filling) and pipe on a touch of whipped cream and a lay on a sprig of mint
pastry cream (or even vanilla pudding) topped with dulce de leche (it's a Latin thing - thick, caramel sauce that's to die for and you can find it in a jar at most markets) then a rosette of whipped cream and a sprinkling of crushed toffee
or a dollop of pastry cream or pudding topped with the aforementioned berries/liquour/cream/mint
a layer of ganache, chopped marachino cherries soaked in amaretto (then drained), whipped cream and an artistic drizzle of melted ganache - maybe some toasted almonds
HTH 
This is the information I have on tassie fillings
From: Cyndi
Lemon Tassies
3 T. fresh lemon juice
3 T. butter, melted
2 large eggs
2/3 c. sugar
Mix all ingredients in bowl. Spoon into unbaked shells. Bake 35-40 min. Remove from pan while still warm. Cool on wire rcks.
Makes about 2 doz.
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From: Cyndi
Brownie Tassies
1/2 c. semisweet chocolate chips
2 T. butter
1/3 c. sugar
1 egg, beaten
1 tsp. vanilla
whole filberts
Melt chips and butter in saucepan over low heat. Remove from heat. Stir in sugar, egg, and vanilla. Place a whole filbert in center of each pastry-lined cup. Top with 1 round tsp. of chocolate mixture. Bake as directed. Makes about 2 dozen.
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CRANBERRY-PECAN TASSIES
1 large egg
3/4 cup brown sugar
1 tablespoon butter -- melted
1 teaspoon finely shredded orange peel
1/2 cup chopped cranberries
1/2 cup chopped pecans
Stir together egg, brown sugar, 1 tbsp. butter and orange peel in a small bowl til smooth. Stir in cranberries and pecans. Spoon about 1 tbsp filling into each cup, stirring occasionally.
Bake at 325 for 25-30 minutes or until filling is set.
Cool on rack. Remove from pans, cool and store.
Source: "Better Homes and Gardens"
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Apricot Pecan Tassie Filling: (for 24 Tassies)
3/4 cup packed light brown sugar
1 egg
1 tablespoon butter or margarine, softened
1/2 teaspoon vanilla extract
1/4 teaspoon salt
2/3 cup finely chopped dried apricots
1/3 cup chopped pecans
Combine the brown sugar, egg, butter, vanilla, and salt in a medium mixer bowl.
Beat at medium-high speed until smooth.Stir in apricots and pecans.
Fill cups and bake at 325° F. for 25 minutesor until set. Cool in pans on wire racks.
A "The Pampered Chef" recipe.
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Chocolate Pecan Tassies Filling: (for 24 Tassies)
1 tablespoon butter or margarine
1 square (1 ounce) unsweetened chocolate
3/4 cup packed brown sugar
1 teaspoon vanilla
1 egg, beaten
Dash salt
3/4 cup chopped pecans.
Melt butter and chocolate in saucepan over low heat. Remove from heat. Blend in sugar, vanilla, egg and salt; beat until thick. Stir in pecans. Spoon about 1 teaspoon filling into each unbaked pastry shell.
Bake at 350° F. for 20 to 25 minutes or until lightly browned and filling is set.
Cool in pans on wire racks. Remove from pans; store in airtight containers.
A favorite of St. Paul's Episcopal Cathedral in Oklahoma City, OK
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From: Anna/Ontario
ALMOND-RASPBERRY FILLING: 24 tassies
Beat 1 egg, 1/2 cup sugar, and 1/2 cup almond paste, crumbled, in a bowl. Divide 1/4 cup red raspberry preserves among pastry-lined muffin cups (about 1/2 teaspoon each). Spoon 1 level teaspoon of the almond mixture over preserves. Sprinkle with chopped sliced almonds. Drizzle cooled baked tassies with additional red raspberry preserves, if desired.
CHESS FILLING: 24 tassies
Stir 1 beaten egg, 1 egg yolk, 1/2 cup sugar, 2 tablespoons melted butter or margarine, 4 teaspoons milk, 1 teaspoon cornmeal, 1/4 teaspoon finely shredded lemon peel, and 1 teaspoon lemon juice in a bowl. Garnish each cooled baked tassie with red and green candied cherries, if desired.
CREAM SHERRY-ALMOND FILLING: 24 tassies
Beat 3/4 cup sugar, 2 tablespoons butter, 1 tablespoon all-purpose flour, and 1/4 teaspoon nutmeg in a bowl. Beat in 2 eggs and and 2 tablespoons cream sherry, stir in 3/4 cup ground almonds. Divide 2 tablespoons strawberry or raspberry jam among pastry-lined muffin cups (about 1/4 teaspoon each). Top with about 1 tablespoon filling.
LEMON-COCONUT FILLING: 24 Tassies
Beat 2 eggs, 1/2 cup sugar, 2 tablespoons melted butter or margarine, 1/2 teaspoon finely shredded lemon peel, and 1 tablespoon lemon juice in a bowl. Stir in 1/4 cup flaked coconut.
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Hope these give you some ideas!
I did several hundred of those for a wedding last month..
Pecan "pies" (I used the crushed pecans)
Berry medley (a dollop of custard with fresh berries on top)
Chocolate peanut butter (I used a whipped ganache, and piped it into the shells)
The possibilities are endless. The shells make any presentation pretty.
This is one of my favourites:
Blend one can of sweetened condensed milk with 1/4 cup lime juice and the zest of one lime until it's thickened. Then fold in 1 1/2 cups Cool Whip. Fill baked tart shells with this mixture, and then top with fresh berries, peaches, or fruit of your choice. My personal favourite is a mixture of strawberries, raspberries, and blueberries. Frozen, thawed berries can be used, also.
You can pipe a star of Cool Whip on top of the berries, for extra prettiness.
Delicious! (Great filling on baked puff pastry, also!)
You can substitute lemon juice and zest for the lime, also.
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