To replace Internet Explorer, try Konqueror. If you prefer to surf the Web in Firefox or Opera, switching to Linux will have little effect on your online life: Both browsers exist in native Linux versions and behave almost the same under Linux as they do under Windows. Not so with Internet Explorer, which is a Windows-only application.
If you use the KDE desktop, the Konqueror file browser doubles as a Web browser, letting you jump from files to Web sites without having to wait for a browser to launch, the same way Explorer can. (Konqueror also runs under Gnome, but may start up more slowly when you first launch it.)
Addicted to AIM? Check out Pidgin or Kopete. AOL offers a Linux version of its ubiquitous instant messenger, but why limit yourself to just one network? Pidgin (formerly known as Gaim) lets you connect with friends on more than a dozen IM networks, including AIM, Google Talk, ICQ, MSN Messenger, and Yahoo Messenger.
If you use KDE rather than Gnome, you may prefer Kopete, which offers similar IM network support but integrates better with the KDE environment.
BitTorrent users, try Azureus. Peer-to-peer file sharing isn't just for pirates these days. Many Linux distributors now offer .iso disc images of their wares via the BitTorrent network. The Azureus BitTorrent client lets neophytes find and share files, but also incorporates advanced features for experts.
Skype runs in Linux, but you could use Ekiga instead. Skype users, fear not: Skype for Linux 1.4 is nearly the same as the Windows version, lacking only video support (as we went to press, a beta version of Skype for Linux 2.0 included video support).
Despite Skype's popularity, however, you may want to use an alternative open-source Voice-over-IP tool called Ekiga. Using the SIP standard, Ekiga lets you talk to other users of SIP-compatible VoIP programs (including Windows Messenger), and it allows you to make calls to landlines if you subscribe to a SIP-compatible VoIP provider such as Gizmo or Wengo. Unlike Skype, Ekiga doesn't use your PC to handle other callers' traffic.