Free, but useful software to enhance productivity

The software that comes bundled with your computer only provides you with a certain and limited set of features, however, there is SO much more software you can buy and get for FREE that will enhance your computing experience. There’s a lot of online applications (Gmail, Blogger, Bloglines) but also many software programs that you can install on your computer.
USA Today writes I’ve been running a PC at work for sometime, though, often getting requests to open such-and-such a file, or to go to a website that couldn’t be viewed on the other reporters’ computers. I’ve also accumulated several useful pieces of software that I’ve mentioned to colleagues, usually when explaining how I did something they couldn’t do on their home PCs.
Now that everyone is running Windows, people have come to me and asked about those things I mentioned; they can run the programs now.