Open source software:

Accounting:
TurboCASH - Operating System: Windows
Replaces: QuickBooks or Sage Peachtree

GnuCash - Operating System: Windows, Linux, OS X
Replaces: Quicken

Edoceo Imperium - Operating System: OS Independent
Replaces: QuickBooks or Sage Peachtree

LedgerSMB - Operating System: Windows, Linux, OS X
Replaces: QuickBooks or Sage Peachtree


Backup:
Areca Backup - Operating System: Windows, Linux, OS X
Replaces: NovaBackup

Bacula - Operating System: Windows, Linux, OS X
Replaces: Simpana Backup and Recovery, NetVault or HP StorageWorks EBS

Amanda - Operating System: Windows, Linux, OS X
Replaces: Simpana Backup and Recovery, NetVault or HP StorageWorks EBS


Compression:
7-zip - Operating System: Windows, Linux, OS X
Replaces: WinZip

PeaZip - Operating System: Windows, Linux, OS X
Replaces: WinZip


Content Management:
Joomla - Operating System: OS Independent
Replaces: OpenText, Sitecore CMS or Kentico

Drupal Operating System - Operating System: OS Independent
Replaces: OpenText, Sitecore CMS or Kentico

Alfresco - Operating System: Windows, Linux, OS X
Replaces: Sharepoint, Documentum, IBM Lotus or Open Text

Get Simple - Operating System: Linux
Replaces: Sharepoint or IBM Lotus


Customer Relationship Management (CRM):
SugarCRM - Operating System: Windows, Linux, OS X
Replaces: SalesForce

vTiger - Operating System: Windows, Linux
Replaces: SalesForce

SplendidCRM - Operating System: Windows
Replaces: SalesForce

ConcourseSuite - Operating System: Windows, Linux, OS X
Replaces: SalesForce


Desktop Publishing:
Scribus - Operating System: Windows, Linux, OS X
Replaces: InDesign, QuarkXPress orMicrosoft Office Publisher


E-mail/Collaboration/Groupware:
Collabtive - Operating System: OS Independent
Replaces: Basecamp

Evolution - Operating System: Linux
Replaces: Microsoft OfficeOutlook

Group-Office - Operating System: OS Independent
Replaces: Microsoft Office Outlook

phpGroupWare - Operating System: Windows, Linux
Replaces:Microsoft Office Outlook or Sharepoint

Thunderbird/Lightning - Operating System: Windows, Linux, OS X
Replaces:Microsoft Office Outlook

Zimbra - Operating System: Linux, Unix, OS X
Replaces: Microsoft Exchange


E-Commerce:
Zen Cart - Operating System: OS Independent
Replaces: Big Commerce, Volusion or Yahoo Merchant

PrestaShop - Operating System: Windows, Linux, OS X
Replaces: Big Commerce, Volusion or Yahoo Merchant

Magento - Operating System: Windows, Linux, OS X
Replaces: Big Commerce, Volusion or Yahoo Merchant

osCommerce - Operating System: Windows, Linux, OS X
Replaces: Big Commerce, Volusion or Yahoo Merchant

Broadleaf Commerce - Operating System: Windows, Linux, OS X
Replaces: Big Commerce, Volusion or Yahoo Merchant

nopCommerce - Operating System: Windows
Replaces: Big Commerce, Volusion or Yahoo Merchant


Enterprize Resource Planning (ERP):
xTuple PostBooks - Operating System: Windows, Linux, OS X
Replaces: NetSuite or Microsoft Dynamics

webERP - Operating System: OS Independent
Replaces: NetSuite or Microsoft Dynamics

dgeERP - Operating System: OS Independent
Replaces: NetSuite or Microsoft Dynamics

Phreedom - Operating System: OS - Independent
Replaces: NetSuite or Microsoft Dynamics

Openbravo ERP - Operating System: Windows, Linux, OS X
Replaces: NetSuite or Microsoft Dynamics

OpenERP - Operating System: Windows, Linux, OS X
Replaces: NetSuite or Microsoft Dynamics

Compiere - Operating System: OS Independent
Replaces: NetSuite or Microsoft Dynamics

ADempiere - Operating System: Windows, Linux, OS X
Replaces: NetSuite or Microsoft Dynamics


Gateway Security Appliances:
Endian Firewall Community - Operating System: Linux
Replaces: Check Point Security Gateways, Barracuda Networks Appliances,
SonicWall Network Security Appliances or Cyberoam Security Appliances

Untangle Lite - Operating System: Windows, Linux
Replaces: Check Point Security Gateways, Barracuda Networks Appliances,
SonicWall Network Security Appliances or Cyberoam Security Appliances


Graphics/Drawing:
Dia - Operating System: Windows, Linux, OS X
Replaces: Visio

Gimp - Operating System: Windows, Linux
Replaces: Photoshop

Inkscape - Operating System: Windows, Linux, OS X
Replaces: Illustrator or CorelDraw


Human Resource Management (HRM):
OrangeHRM - Operating System: Windows, Linux, Unix, OS X
Replaces: Halogen software, iCIMS or Ascentis


Invoicing:
Simple Invoices -Operating System: OS Independent
Replaces: FreshBooks or Bill.com

Argentum - Operating System: OS Independent
Replaces: FreshBooks or Bill.com


Office Productivity:
OpenOffice
Replaces: Microsoft Office

AbiWord - Operating System: Windows, Linux, OS X
Replaces: Microsoft Word

FreeMind - Operating System: OS Independent
Replaces: MindJet MindManager

Gnumeric - Operating System: Windows, Linux
Replaces: Microsoft Excel

KOffice - Operating System: Windows, Linux
Replaces: Microsoft Office

PDFCreator - Operating System: Windows
Replaces: Adobe Acrobat


Point-of-Sale (POS):
Openbravo POS - Operating System: Windows, Linux, OS X
Replaces: Epicor Retail Store or Retail STAR

http://www.lemonpos.org/ - Operating System: Linux
Replaces: QuickBooks Point of Sale or AccuPOS

Floreant POS - Operating System: OS Independent
Replaces: POSito, QuickBooks Point of Sale or AccuPOS


Project Management:
OpenProj - Operating System: Windows, Linux, OS X
Replaces: Microsoft Project or Oracle Primavera

GanttProject - Operating System: Windows, Linux, OS X
Replaces: Microsoft Project or Oracle Primavera


Time Tracking:
TimeTrex - Operating System: Windows, Linux, OS X
Replaces: Replicon or Tenrox Timesheet

eHour - Operating System: Windows, Linux, OS X
Replaces: Replicon or Tenrox Timesheet


Web Design:
Kompozer - Operating System: Windows, Linux, OS X
Replaces: Adobe Dreamweaver or Microsoft Expression Web

BlueGriffon - Operating System: Windows, Linux, OS X
Replaces: Adobe Dreamweaver or Microsoft Expression Web