If you go to the recipe section, you can find a bunch of different recipes for filling. You can do whatever you want - as simple as the same BC you frost your cake with, to jams or jellies, to other fillings that you make separately - like mousse, etc. There are so many possibilities. Or search the forums for "fillings" and you will see many threads that discuss different options.
Just remember, if you use a filling, make a "dam" with an open coupler and whatever you are frosting your cake with. Other forums discuss this too but basically, you are putting a circle of icing in the middle layer as kind of a small wall to keep your filling from oozing out the sides of your cake.
I have recipes I use for fillings such as coconut, pineapple, lemon, cream cheese, caramel, etc.
My favorite filling is raspberry amaretto and I just use a jar of raspberry preserves and add a little amaretto to it. I find preserves makes a very good filling.
HTH 
What did you have in mind?
Here are some previous threads:
http://www.cakecentral.com/cake-decorating-ftopict-51901-fillings.html
http://www.cakecentral.com/cake-decorating-ftopict-51550-fillings.html
http://www.cakecentral.com/cake-decorating-ftopict-49258-fillings.html
http://www.cakecentral.com/cake-decorating-ftopict-48940-fillings.html
http://www.cakecentral.com/cake-decorating-ftopict-48535-fillings.html
HTH
There are recipes in the recipe section on this website, or you could use pudding, frosting, or jam. You can mix a fruit-flavored jam into a buttercream frosting.
For a caramel filling, I've added a tablespoon of brown sugar to caramel flavored yogurt. You could adjust the sugar to your taste. I've also mixed a couple of tablespoons or so of raspberry syrup to chocolate buttercream frosting, and it went over well.
You'll need to make a dam around the outer edge of the layer before you add any filling that is soft -- fill a pastry bag with buttercream icing, use a star tip, and make a ring of icing around the outer edge. Then add your filling, making sure not to let the filling go higher than the dam. I believe Toba Garrett recommends 4 oz. of filling for an average round layer.
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