Broswer: Firefox.
Supports Java applets, Flash, PDF plugin nicely.
Light weight, themeable, tabs support, available for Linux/Win/Mac platform.
[ Screenshot ]

E-Mail: Thunderbird.
Support for IMAP, POP, LDAP, SSL, Offline mails, MIME
Spam Filter rules, Address bar, Themeable.
[ Screenshot ]

IRC-Client: Xchat.
Multiple channel and server support. GTK2 interface.
Transperency support. available for wide range of platforms, even for Windows.
[ Screenshot ]

FTP-Client: gFTP.
Multithreaded ftp client, GTK1/2 interface.
Supports the FTP, FTPS, HTTP, HTTPS and SSH protocol.
FTP and HTTP proxy server support.
[ Screenshot ]

Yahoo Messenger: ymessenger.
Conference, emoticons and P2P file transfer available
All features consistent with Windows yahoo messenger excpet for the Voice chat and Webcam support.(Huge Drawback)
[ Screenshot ]