I've only filled a cake once before. It was a caramel mud cake, and I filled it with (surprise surprise) dulce de luche caramel.

Now I'm making other cakes, but I'm not sure what to fill them with.

When I can't think of a filling, I use the oreo filling recipe (from this site) and it always tastes great. Sometimes I omit the oreos all together, sometimes I replace them with other cookies/candies.
Ganache is another favorite filling of (for so many different cake flavors).

Plain buttercream, buttercream mixed with fruit preserves, jams, jellies, or curds...

Endless possibilities.

I always used plain buttercream, but I wanted to try something new, so recently I taste tested 7 fillings...I made half recipes of each...4 from this site, 1 from another site, and 2 puddings made w/ heavy cream instead of milk...

None were horrible...well, the 1 from another site I didn't care for at all...

But my top 3 choices were the easiest & least expensive to make:
Vanilla pudding made w/ cream
White chocolate pudding made w/ cream
Easy Bavarian Cream: http://cakecentral.com/recipes/7607/easy-bavarian-cream

This was sort of the order I rated them at first, except that really the puddings were pretty equal, but the Bavarian Cream was third...

Then it sat in my fridge for a day, and the Bavarian Cream shot to the top of my list...OMG it is incredible!!
I freakin' loved it, and with chocolate cake...no words!!
I was rationing it so I could have some over a few days, and I swear it only got better the longer it sat!

nnnnoooooooooooooo!!!!!!! Your post was so well written I got all excited about the Bavarian cream. But it contains cool whip, which I don't think is available outside of America. Not fair....waaahhhh!!!

OH NO!!!

That's not right at all!

So many options!! I'm torn between Lemon Cream (a lemon curd lightened with whipped cream) and simply whipped cream sweetened with powdered sugar. To keep it from going back to a liquid state, to keep it stable, I add a little Clear Piping Gel. At this point it's fabulous and lucky to make it to the cake rather than my mouth, but it is kind of a blank palette for other flavors. I'm kind of in love with CK Icing Fruits right now. I'm torn between my favorites- Orange and Raspberry, so I'll just date them both for awhile. The Orange Icing Fruits went into my pancake batter the other weekend, and we had a lovely Sunday morning together. That reminds me- they also bake well, so can be folded into cake batter for a real fruit taste as well.

I'm hungry!!

Oh! Don't forget to make an icing dam to hold all those softer fillings in place! Here's our video on torting and filling: http://www.youtube.com/sweetwiseinc#p/u/1/6oLhbNjMNWM

I love youtube!! And oh man I'm hungry, that cake looked nummy.

Never heard of piping gel, but I just checked, and it's available here, and apparently is useful for water effects too... awesome.

Lordy, those cakes looked pretty flat to start with before she levelled them! I probably would have used them as they were But then again, I've only ever filled one cake so far, and it kept bulging and slipping, so perhaps I ought to pay attention and level more.

I like to use vanilla bavarian cream, dark chocolate mousse and white chocolate mousse. I have also used fruit cloud creams (from the Cake Bible) - strawberry puree, raspberry puree, or lemon curd folded into whipped cream stabilzed with a bit a gelatin. Heavenly.

I have just ordered some hazelnut paste and praline paste. Can't wait to incorporate them into my fillings.

Here in the UK, most cakes are filled with 'normal' buttercream - ie 1 part butter to 2 parts icing sugar.

For vanilla sponge (which is the most popular flavour) you add vanilla extract and you also fill one layer of the cake with seedless raspberry jam.

For lemon sponge, add some lemon curd.

For chocolate, add some cocoa....

Of course, there are some other fillings, but these really do tend to be the 'luxury' lines and are a lot more money.