Recommended Software

Linux has an incredible variety of software. Many of the Programs below are part of two of the most popular Desktops: GNOME, and KDE. I just supply some favorites.
For additional information, check the Linux Wiki table (best), Migrating from Windows to Linux (dated), or this table of equivalents.

Some Helpful / Recommended Software

Type
GNU/Linux *
Windows

Office Productivity Suite

Use OpenOffice.org
webopedia info: LibreOffice

Use OpenOffice.org

LibreOffice is a FREE comprehensive Office Suite - for Linux, Windows, even Apple (NeoOffice)! (OpenOffice is the Oracle/Apache version used by many businesses.)

Abiword is an elegantly simple Word Processor; and Gnumeric is a powerful and easy to use Spreadsheet from GNOME Office.

KOffice (part of KDE) is maturing as well.

There are Windows versions of LibreOffice, OpenOffice, and Abiword. There is even a project to bring KOffice (and the whole KDE Desktop!) to Windows (KDE on Windows).

Web Browser

Mozilla Firefox

Firefox blows IE away! ... tabbed browsing and intutive control of cookies, etc. There are versions for Linux, Windows, Apple, and others!

Chrome and Chromium

Opera has a different style, but you might like it.

Firefox, Chrome, and Opera are also available for Windows.

Java plugin for browser

Email Client

Mozilla Thunderbird

Mozilla Thunderbird
attractive interface with spam filters and GPG

Kmail (part of KDE's Kontact PIM)
great GPG integration
Sylpheed – fast, light Win32 version too)
Evolution | Evolution (GNOME) | openSUSE link– PIM (in distros) Outlook and Groupwise protocols support

Thunderbird is recommended
Sylpheed is available,
and Kmail/Kontact may be on the way.
Remember Eudora? It is coming back as Open Source Eudora OSE | Penelope project.

Instant Messaging client

kopete (KDE)

Psi (neat, cross platform!!)

Pidgin amazing cross platform, multiprotocol client (GNOME)

Psi is excellent.
Pidgin has come a long way since gaim superseded "AOL Instant Messenger."

Newsreader

KNode (also part of KDE's Kontact PIM)
or Use Thunderbird (see above).


Use Thunderbird (see above)

FTP client

gFTP (GNOME)

old trustworthy WS_FTP_LE, OR try SmartFTP

PDF Reader

Adobe Acrobat Reader)

For linux, there are many other utilities, including: KDE Okular, GNOME Evince, Ghostview (ggv), kghostview, gpdf, kpdf, and xpdf. These may do additional things and/or be faster.

use Acrobat Reader, or search other options - incl Evince

To make PDFs

This functionality is built in to KDE printing, Libre/Open Office, Abiword, etc.!
other methods include using Ghostscript.
ps2pdf is an online (platform agnostic) front-end for Ghostscript
CUPS-PDF is another nifty utility.

Built in to Libre/Open Office!
Also may try printer driver type utilities: doPDF, PDFCreator, BullZip PDF Printer, ... Commercial products: Click2PDF, PDF995, Adobe Acrobat (see price!).

Graphics

Gimp is the Swiss Army knife of bitmap graphics programs, with the functionality of Adobe Photoshop.

For linux, there are various other bitmap and vector drawing programs. Vector programs include LibeOffice.org's LO Draw | OpenOffice.org's OO Draw, as well as Inkscape and Krita (KDE).

There is version ofGIMP for Windows.

IrfanView (alt) - Freeware: nifty, compact.

Archiver / Zip utils

Ark, File Roller, etc. usually come right in the standard distribution installation.

7-Zip, or WinZip(commercial)

Firewall

built-in tools with most Linux distributions. Other graphical options include Firestarter.


ZoneAlarm - highly configurable (free version)
Windows firewall has minimal functionality.

Antivirus

Less need for it ... but we sometimes do need to protect our Windows-using neighbors, lest we inadvertently pass one on.
See ClamAV
or Avira Free Antivirus, or AVG Anti-Virus (Free edition)

Anti-Spam

Very intuitive "trainable" Junk Mail filters are now built into the Mozilla Thunderbird mail program!

SpamAssassin is a really smart program: a little tricky to configure, but it can integrate easily with Kmail or other mail programs!

best to just use Mozilla Thunderbird's built in trainable "junk mail" filters.

Anti-Spyware / Anti-Malware

Don't need it, unless you run "root" (administrative user) all the time and use no discretion for installing code! ...

Ad-Aware's free edition, Spybot ...

Note to Windows users:
Do not ... Do not! ... DO NOT!! run always as administrative user!

This list could be made longer of course!