Balanced binary tree implementation

[Lucas P Melo]

Hello,

I would like to use a balanced binary tree implementation (preferably 
within some API).
Any hints about where I could find it?

I am looking for something that implements insertion, deletion, search 
and a special search that returns the lesser element bigger than a given 
key [1].

A nice possibility would be an extensible API that allows me to inherit 
its classes and to add operations myself.

Thanks in advance.

[1] Ex: 1 2 3 4 5 6 are elements of the bbt. If I use this operation 
given 4 as the parameter, the value returned would be 5.