I really really need a good idea for a filling for a plain old yellow cake. Anybody got any ideas? I want to make it taste gooooooooood!

The great thing about a "plain old yellow cake" is that it goes great with pretty much any filling!

You can fill with a different flavor icing than what you're frosting the cake with:
Vanilla Buttercream
Almond Buttercream
Hazelnut Buttercream
Lemon Buttercream
Orange Buttercream
Mint Buttercream
Crème Bouquet Buttercream
Crème Royal Buttercream
Brown Sugar Buttercream
Fluffy Meringue
Whipped Cream
Vanilla Bean Buttercream
Lemon Buttercream with Lemon Zest
Orange Buttercream with Orange Zest
White Chocolate Buttercream
Chocolate Buttercream
Chocolate-Almond Buttercream
Chocolate-Orange Buttercream
Chocolate-Mint Buttercream
Coffee Buttercream
Mocha Buttercream
Peanut Butter Buttercream
Dulce de Leche Buttercream
Raspberry Buttercream
Strawberry Buttercream
Peach Buttercream
Mango Buttercream
Guava-Passionfruit Buttercream
Key Lime Meringue
Orange Whipped Cream
Chocolate Chip Buttercream
Chocolate-Chocolate Chip Buttercream
Mint-Chocolate Chip Buttercream
Peanut Butter-Chocolate Chip Buttercream
Raspberry-Chocolate Chip Buttercream
Orange-Chocolate Chip Buttercream
Peppermint Stick Buttercream
Cream Cheese
Fudge
Coconut-Pecan
White Chocolate Cream Cheese
Orange Cream Cheese

Or you can do a fruit filling:
Strawberry
Raspberry
Blueberry
Cherry
Apricot
Peach
Apple
Cranberry
Pineapple
Pomegranate
Guava
Mango
Passionfruit
Fresh Peaches
Fresh Strawberries
Fresh Raspberries

A curd:
Lemon Curd
Lime Curd
Orange Curd
Raspberry Curd

A custard, Bavarian, or mousse:
Vanilla Custard
Chocolate Custard
Vanilla Bavarian
Cookies n Cream Bavarian
Coconut Bavarian
Chocolate Bavarian
Peanut Butter Bavarian
Milk Chocolate Mousse
Dark Chocolate Mousse
White Chocolate Mousse
Strawberry Bavarian
Raspberry Bavarian
Peach Bavarian
Cherry Bavarian
Banana Bavarian
Pineapple-Mandarin Orange Bavarian
Chocolate Almond Cherry Chip Bavarian
Pistachio Cream
Spumoni Fruit Cream
Raspberry Mousse
Strawberry Mousse
Lemon Chiffon
Mascarpone Custard
Cannoli Cream
Cheesecake Cream

A ganache:
Milk Chocolate Ganache
Dark Chocolate Ganache
White Chocolate Ganache

Or:
Coconut-Pecan
Penuche
Penuche Pecan
Nutella
Dulce de Leche
Poppyseed