Here are a few favorites. These are not try then buy, nor do they ask for your credit card. This are high end programs at no cost, great if your budget is on a shoestring.
Gimp is an open source program found at http://gimp.org/about/ Nicknamed the free photoshop. Takes time to learn but a fun program. Mask, layers, shadows, makeseamless (not sure photoshop has that one) Does some pretty cool stuff. I use this one the most, still learning all its features. Supports many file formats.
inkscape http://www.inkscape.org/ open source vector graphics editor, similar to illustrator, coreldraw. My daughter Jessica loves this one. She loves to draw and is now drawing on the computer. Just ask and I’ll have her show you the thankyou cards she designed.
Scribus http://www.scribus.net/ open source page layout, makes your work press ready
Open Office http://www.openoffice.org/ open source office suite, does much, converts your documents to pdf files