How to tamper the network data between the client application and server on Mac OS x?

I am working on Mac OS X.

I have a client side application (Example: client.app) which has a inbuilt browser (for login and sigout etc., ).

I have to test the network data from client.app to its Server. (Like tamper data)

Can anyone suggest some tools for tampering data from the client.app to Server?

Exactly like Tamper Data Addon in Mozila.

Kindly post some useful tools for testing. (it should cover functional, stress, security testing as well)

Thanks in Advance.