Note that you need Avahi for all these
FileSharing (just works!)
You can use gshare, it's available on Ubuntu and Debian on APT.
To make it work just run gshare-manager and enable access, note that this will share a dir on your home dir called Shared Files
To browse files go to Nautilus Network Location
So it just works!!
MusicSharing (just works!)
Just use Rhythmboxs DAAP plugin, it's available on every Rhythmbox install (please confirm this).
Simply use Rhythmbox to share and read.
Instant Messaging (secure and automatic!)
This is reeeeeeeeeally cool.
Qué guay! Cómo mola! --SilviaMiranda
Oh por Dios!! --DiegoEscalanteUrrelo
Just add an account of type Bonjour and that's it!!. Debian's Pidgin build just works. Ubuntu's Pidgin (feisty's one) doesn't have the Bonjour account plugin, you have to install packages from:
Lennart (Avahi hacker) says it just works™.