If you ever work with photos or other images, GIMP is a must-have alternative to Adobe's expensive Photoshop software. GIMP is great for making Web graphics, creating marketing materials, and retouching product photos, and any images you create can be saved in numerous formats, including Photoshop. Once you try out GIMP's high-end graphics editing and creation features, you'll be amazed that it doesn't cost anything.
Finally, whether you're running Windows or not, it's a good idea to have some sort of antivirus protection in place, and ClamAV is a great free choice. The popular cross-platform tool detects Trojans, viruses, malware, and more.