HTTP authentication software

Hi,

Are there any software that performs HTTP authentication?  In particular, I
have an server application that resides on a web server, and I want to write a
non-browser client application that can talk to the server app.  I'm concerned
about how my client app can authenticate with the web server.  If there are
existing software that I can use, that would be great.  Otherwise, is it
difficult to write my own?

thanks,

--Michele