Vicidial / Asterisk is de facto in call centers, more than 90% call centers who happen to use a proper dialer use vicidial only.
LibreOffice: www.libreoffice.org is powerful alternate to microsoft office can be installed on Windows, Linux and Mac operating system. Using Libreoffice you can still make microsoft office files like .doc or .xls you can also import same MS office file to do further work on them. Definitely a must-have for open source lovers.
Nagios http://www.nagios.org/ should be in your call center software list if you care about your IT infrastructure (for sure you do care 😛 ) you can Monitor your entire IT infrastructure and detect security breaches consequently you can reduce downtime and business losses.
Finance / Accounting
Front Accounting http://frontaccounting.com/ is a web-based complete accounting software which will cover nearly all process of accounting just like an ERP very simple to setup and use good thing about this software it is very much compatible with Pakistan Tax and Accounting rules and regulations. It is not core call center software but it helps your call center do care of finance and keep things transparent for auditing. no matter you have small or big call center it is equally good software.
Vtiger www.vtiger.com web-based CRM software for call center, Automate your sales, marketing and support process for your own clients also integrated email, contact and leads management system. a good CRM like Vtiger will save your time working on tons of other software and messy excel sheet for many things you do. Moreover vtiger can be integrated with Vicidial.
Projectforge www.projectforge.org Working on a project ? Like hiring people, assigning duties, or managing teams ? Well Projectforge is for you! Since it is web-based system so your teams of workers can work from other location offices as well. Vtiger lacks project management module however both are good software for call center technology.
There is tons of software out there but software I mentioned in list above will boost your bottom line productivity. Beside this you can use other opensource software like ubuntu as OS (agents wont like it though 🙂 ), Mozilla Firefox browser, Thunderbird email client can also be integrated with Vtiger CRM.