Re: Open source programs
PdfXchange is open source? I think it is free, but is not open source, isn't it?
If we should mention only open source, then
Zotero is a lovable piece of software.
Notepad++ is just perfect IMHO

It can highlight syntax of many coding languages, and has great features.
GimpShop is Perfect again
OpenOffice,
StarOffice, and their siblings. Being free while being able to provide almost everything provided by Office (and sometimes more).
Miro is perfect too. It can play almost every format of media, even the streamed ones! Very very handy.
If we can recommend freeware programs (regardless of their source availability),
Opera is the absolute leading browser in terms of innovation and features and functionality. Many of the features we see in everyday browsing experience (such as tabbed browsing and many others) are first invented by Opera. It is the fastest browser ever, and can eat all other popular good browsers such as Firefox, Chrome, and Safari for its breakfast. It is extremely compatible with many sites, and passes ACID tests (something Chrome and Firefox and Safari fail to). I think since it is originated in Europe, it is underestimated, and deserves much better attention. Its only weak point was its inability to use extensions. This problem was solved after being requested for hundreds of times by community members, and it is catching up in that matter.
Avira Antivir Free (free but closed source) is just perfect. It is the best free antivirus. It doesn't eat up system resources (CPU/RAM) like does the other bloatware antiviruses (almost all of them), while still providing very strong security measures. I have personally tested it, and it beats every competitor IMHO.
Bit Defender Free is very good too. Its free version has some intense limitations though.
Another Very handy free software which can and should replace the usual image browser of the systems, is
FastStone Viewer. I can't describe how it can facilitate your image browsing experience. It has several other features as well.
Although trinker already stated, I would like to remention it.
PdfXchange is the best ever PDF reader/writer. I for one give it 100, while giving Adobe Acrobat 70/100, Foxit 50, and Adobe Acrobat Reader 20/100.