This really doesn't concern cartographic software, so it doesn't really fit in the software discussion forum. I posted this list in my Facebook profile, but I'm interested in what y'all say.
Oh, and please note which operating system you use.
My list of essential software (Mac)
I've left out programs that come with Macs - like Mail and Address Book. This is a little list of software that everyone should have. They're all useful programs that I use on a daily (or near daily basis). Of course, the few programs might have made the list because they're free...
Yojimbo - best $39 I've ever spent. I couldn't live without this program. Have a scrap of information that you need to write down, but it doesn't warrant its own document? Yojimbo is great for keeping track of notes. Image files that don't belong in iPhoto (like artwork)? Yojimbo! Want a hard copy of a website you refer to all the time? Yojimbo! A lot of PDF files to keep track of? Yojimbo! They offer a free 30 day trial.
Quicksilver - hard to explain. Its free, so you might as well try it. I had it for about a month before I started trying to figure it out. Every few days I'd play with it for a few minutes, and then give up. A few days ago, in a eureka moment, I understood how it works, and I use it all the time now. Check out their website, or this wikipedia article.
Office 2008 for Mac - who doesn't need Word, Excel, and Power Point? Check out Open Office for a free (somewhat buggy) alternative. ($150-ish)
Opera - a free web browser. I really liked the interface, but Facebook doesn't like it. Several other sites also seem to have trouble loading in it, so I've switched back to Firefox. If companies made plug-ins for Opera, I'd use it anyway, but few companies bother. They only do Internet Explorer, Firefox and Safari.
One feature that Safari has that the others don't, and I wish they did, is that if you note a web address in your address book, it would show up in safari's bookmarks under the name of the address book entry.
Opera - http://www.opera.com/
Firefox - http://www.mozilla.com/en-US/
Notebook - when I switched to Macs from PCs I had a minor breakdown when I realized that Microsoft OneNote was not available for Macs. Between Yojimbo and Notebook, I've largely come to a happy medium. Notebook is a tabbed writing program that mimics, strangely enough, a notebook. I write all my notes here first, and use it as a journal. ($49.95)
Gimp - does almost everything that Adobe Photoshop ($699) does, but is priced about $700 less, free in other words.
Inkscape - an free alternative to Adobe Illustrator($599). I've come to really like vector graphics, and I'm looking forward to using this software more.
Scrivener - a tool for authors. A great application for drafting prose. ($39.95, free 30 day trial)
Scribus - a desktop publishing program. Alternative to Adobe InDesign ($699) or Microsoft Publisher ($169). Its free!
Skim - a free PDF viewer. More functionality than the equally free Adobe Reader. Allows you to make bookmarks within PDFs, take notes, etc. Still won't allow you to fill in forms and save the information if the PDF author doesn't want you too. Damn them.
FreeMind - a mind mapping program. I don't really understand how to use it yet, but the mind maps I saw made with Curio were really cool. Too bad Curio is expensive ($99).
Curio - http://www.zengobi.com/products/curio/
FreeMind - http://freemind.sourceforge.net/wiki/index.php/Main_Page
FastScripts Lite - a tool that allows you to assign hotkeys to scripts. Very useful if you're script happy (like me). The free version only allows you to assign hotkeys to 10 scripts, but with Quicksilver, you don't really need any more. The pay version is $14.95.
Delicious Library - There are a bunch of programs out there to keep track of your books, compact discs, and DVDs, but as far as I know, only Delicious Library can use your webcam to scan their bar codes. Just hold the book/CD/DVD up to you webcam and Delicious Library scans the bar code and looks it up online. It also can look up electronics, power tools, and board games. Anything you can buy at Amazon, Delicious Library can catalogue. Have an easy to maintain list of your possessions for when the insurance people need one. ($40)
AddressBookSync - a free utility that downloads peoples profile pictures and birthdays from facebook to your address book. Only does folk you already have in your address book, and it follows Facebook's guidelines on such things. Pretty cool. Now if only they'd allow for scheduling they sync.
Name Mangler - a free bulk renaming utility. Very useful.
OnyX - a system utility to run various maintenance tasks. Don't know how useful it really is, but I like having it around, and I use it about once a week. Its free too.