Here are eleven pieces of free software I use pretty much every single day. Some of these come in free and paid versions - in each case, I've become such a loyal user of the software that I'm now a paid user, as I'm a big believer in paying for what I use.
Xmarks - http://www.xmarks.com/
Xmarks enables me to access all my bookmarks on all of the computers. It works with Firefox, Internet Explorer, Safari, and Google Chrome. Even my iPod Touch has all of my bookmarks. Picasa
Picasa - http://picasa.google.com/
I'm starting to build up an image archive of photos taken with my own camera for various projects. Picasa has become my organizing tool, as it keeps them all organized and easy to find, enables me to do simple touch-ups , tagging, and easy web sharing, though I still use Flickr for most sharing purposes.Skype - http://www.skype.com/
I use Skype for my business-related calls. Most of the features are free.
Dropbox - http://www.dropbox.com/
I often use this for sharing files between various computers, as well as sometimes making files available to others that I want to share with. While I have home networking set up, it isn't seamless and it doesn't allow retrieval from anywhere, so Dropbox fills in that gap incredibly well.
Remember the Milk - http://www.rememberthemilk.com/
Remember the Milk handles my to-do lists for me. I usually use RtM hand-in-hand with processing my "inbox" (mail, notes to myself, and so forth). If there's a task I need to do, I put it into RtM, and when I need to get down to business, RtM simply has my to-do list ready to go for me. I've used it for years.
Gmail - http://gmail.google.com/
The best thing I ever did was consolidate my email inside of Gmail. It enables me to effortlessly search through all of my email from any web browser, which is incredibly useful for both personal and professional things. I often just email things I need to remember to myself so I can search for them and find them later on within Gmail.
Gcal - http://www.google.com/calendar
This serves as an incredibly effective personal schedule for me, enabling me to quickly see (from almost anywhere) what's going on today, tomorrow, this week, this month, and so on. Recurring appointments, the ability to color-label different kinds of things, and the plethora of different views just a mouse-click away makes this an essential tool for me.
Digsby - http://www.digsby.com/
I use Digsby on any Windows-based computer I'm on to keep up to date with Twitter, Facebook, and the instant messaging programs I use all at once. Whenever something new comes along, it pops up in the corner and I can click on it to reply if I so choose. It's great to have on if I'm just searching for ideas and it turns off with just a click when I need to focus.
Notepad++ - http://notepad-plus.sourceforge.net/
Notepad++ is what I use when I'm writing posts (this article was typed out in Notepad++), writing code, or trying to organize ideas. It does so many little things better than the default Notepad that I consider it essential. I even type out long emails in it, copying and pasting when I'm finished.
Google Docs - http://docs.google.com/
When I'm assembling longer documents, doing basic spreadsheet work, or assembling presentations, I usually use Google Docs. Doing this allows me to work on those documents no matter where I am. Even better, you can effortlessly share these documents for collaborative purposes, allowing other people access to the document and allowing them to make changes and keep track of them.
Google Reader - http://reader.google.com/
I try to keep track of different blogs. I use Google Reader to do this - it simply shows me the latest posts from all of these sites at once. I can group these sites into whatever collections I like and it keeps track of which ones I've read and which ones I haven't.
This list pretty much sums up the software I use - and it's all FREE.