I have talked before what a fan I am of open source software. Here’s a list nice list: 21 Awesome (But Lesser Known) Open-Source Applications For Windows. Fairly surprised Filezilla is on this list, I would call that one of the better known open-sourch apps.
Not on the list, but one I have started using again: Pidgin a GTK+ instant messaging application for Windows and Unix. It supports AIM, ICQ, Jabber/XMPP, MSN, Yahoo!, Bonjour, Gadu-Gadu, IRC, QQ, SILC, SIMPLE and more. Basically one instant message client for all your needs. Also comes with an optional spell checker. Great for people like… you know, me.