Two years ago, the small business where I work would never have considered selling Linux systems. When it comes to computers, the vast majority of our customers are only familiar with Windows. Trying to sell them an Ubuntu system would have been similar to what Sisyphus experiences with his boulder.
Times have changed, however, and now it's a bit easier - especially since I can demo a system and show that they'll be able to do many of the same things even with a different OS powering the computer.
Common Windows applications that are available for Linux
My average customer wants to do a few key things with their system and not much else - download music, chat, send email, store digital photos, and play some casual games. Many of their preferred Windows programs are also available for Linux.
Limewire - Just because you're switching operating systems doesn't mean you have to give up downloading music. Limewire is built on Java and is available for Linux.
Picasa - Most of us have digital cameras by now, and many of us have hundreds (if not thousands) of digital photos. You may have grown to love Picasa on Windows, but you won't have to give it up if you switch.
Skype - Keeping in touch with family members is important. Doing it for free? That's a bonus, and Skype will let you do that, even on your new OS.
Google Gadgets, Google Desktop - A number of my users have wound up with these on their windows system thanks to the Google Toolbar (which you can also add to Firefox). If you enjoy using them, you can install them on Linux, too.
Flash - No one wants to miss out on their favorite casual games (like our Time Wasters). Adobe's Flash runs on Linux, too, and it's powering the vast majority of those addictive little distractions.
Apps you might not know, but you'll love after the switch
Not everything you run right now is available for Linux. There are, however, tons of great desktop and web applications that can fill in nicely. Better yet, nearly all of them are totally free!
Disc Burning: Brasero (Pictured), K3B
Linux users want to be able to burn stuff, too, and they have good programs to do it that are just as easy to use (if not a little easier). There is actually a Nero version for Linux, but Brasero and K3B are excellent, free replacements.
MySpace, Facebook, and more - all in the same program. Meebo and MSN Web Messenger run in your web browser, so you'll be able to run them as well.
iTunes: Amarok(pictured), Songbird
Apple isn't quite ready to get on board the Linux train just yet, but Amarok and Songbird both provide very strong alternatives to iTunes. You won't be able to buy from the iTunes store itself, but there are plenty of other places to purchase new tracks online.
Internet Explorer: Firefox, Opera(pictured)
Likely the biggest switch you'll be making is your web browser, but it doesn't have to be hard. You can customize Firefox's appearance so it looks almost exactly like Internet Explorer if you start to experience withdrawals - Linux.com has a good writeup on how to do it. If you really, really must have the original, you can install Internet Explorer in Wine - though since you're changing operating systems you may as well try out a more feature-rich browser like Firefox or Opera.
Watching Movies: VLC
I've never been too excited by the programs makers ship on their systems for watching movies. VLC is an excellent player for both downloaded and DVD movies, and you won't have to hunt around for codecs like you may have with Windows Media Player.
Torrent Downloads: Transmission, Vuze (pictured), Limewire
More and more of my customers are getting the hang of torrent downloading. If you want a basic, torrent-only program, Transmission is a solid choice. If you're already using Limewire, its new version can handle torrent downloads as well so you don't actually need a separate program. Video junkies should check out Vuze, which provides additional media functions you'll love.
Office: Openoffice.Org, Zoho Office, Google Docs
As is the case with Internet Explorer, Microsoft Office doesn't ship for Linux either. However, both Zoho and Google provide excellent alternatives that run in your web browser and OpenOffice.Org is available for those who prefer running programs on their desktop. All three are excellent replacements.
Games: AisleRiot Solitaire (pictured), Mahjongg, Sudoku
Gnome Games contains tons of excellent games - you definitely won't miss your old Microsoft stand-bys once you've made the jump. Aisle Riot runs circles around MS Solitaire. Some other games you'll get by default that you don't on Windows: Mahjongg, Sudoku, Blackjack, Tetravex, and GnomeTris (a Tetris clone).