where can I get a free package that I can integrate with my site I am building now?

Would like it to be easy to use, but at the same time, flexible as:
1- Can recognize and use my own authentication capabilites without relying on its internal user authentication (so I do not have two authentication systems for my users)
2- Can change its themes, and at the same time, can integrate messages viewing into my site (i.e.: does not need to be a seperate site or page feel (without my own menus..etc)

Please help...

If you think a package I can use, though does not have the full features above, still give me the link to add it into my considerations-list..

Thanks in advance...