HamQSLer is the premier free, open source program for designing and printing your QSL cards. HamQSLer allows you to design professional looking QSL cards, and then either print them with QSO information added on your own printer, or save them to JPEG files for forwarding to a QSL print shop. HamQSLer has a number of features that are not available in any other QSL design program; for more information on the program, see the HamQSLer section of this website.
As a preview of what your card could look like, see the image below. This is the card you will produce by following the Getting Started chapter of the user manual included with the program. Some sample cards produced are shown on the Sample Cards page in the HamQSLer section of this website.
HamQSLer is an extremely powerful program that allows you to add multiple images and text items to the card, and provides features such as macros, but it is also easy to use. You could probably design your first card within the first 15 minutes just by poking around in the program, but you should follow the Getting Started tutorial in the user manual to get a better idea of what the program can do.