Everyone is writing "Foo Best" lists all full of good Linux apps, so here are my own 7 Best Excellent Linux Apps You May Not Have Been Introduced To Yet. They are presented in no particular order or categorization, they're just good applications I've been using and enjoying. They are all 100% genuine Free/Open Source software and not crusted with proprietary baggage, and available via the usual distribution package managers.
1) GLabels: Label, Business Card, and Postcard PrinterOpenOffice Writer has a pretty nice labels template, but Glabels is faster and easier. You can make file labels, name badges, CD and jewel case labels, business cards, address and shipping labels-- you name it, if it's an Avery label Glabels supports it. Avery labels are pretty much the de-facto standard, and it has a nice wizard for creating custom templates. The only label type it doesn't support are the little specialized roll label printers, like Brother label printers.
2) Kile: Integrated Graphical LaTex EnvironmentI've been using Kile to write my latest book (Build A Computer Sound Studio With Audacity! No Starch Press! Coming Soon!) and it is a joy to use. I'm happy to have a publisher that doesn't require dumb icky Microsoft Word docs, and LaTex isn't hard to use when someone else writes the stylesheets; all you need to learn is the markup. Kile has syntax highlighting and lots and lots of handy pointy-clicky functions. It also permits as much manual markup and futzing as you want.
3) recordMyDesktop: Desktop Video RecorderIf you've been watching Linux desktop video howtos and wondering how they were created, it may be that they used recordMyDesktop. recordMyDesktop captures everything you do on your Linux desktop. It has a simplified graphical interface, and if you want to fine-tune and tweak it also has a comprehensive set of command-line switches. It can also capture audio, and for the best results I recommend getting acquainted with JACK (the Jack Audio Connection Kit), because this gives you the most control and flexibility. JACK lets you route any audio output to any input, so on most Linux systems it's the only way to capture system sounds.
You could also try audio routing with PulseAudio, which is finding its way into more distributions, but for me it's been unreliable. If your system already has it, try it first.
4) K3b: CD/DVD Writer and RipperOK, so K3b (KDE burn, baby, burn) is well-known, but it still deserves as many praises as it can get. It is a beautiful graphical front-end to the giant host of commands that are needed for CD and DVD writing and ripping. It even queries CDDB (Compact Disc Database) to fetch song titles, disc, and artist names automatically. It supports CD Text, which is an extension to the universal Red Book CD Audio standard that displays titles and names, and it supports the newfangled mixed-media CDs that combine audio and video tracks. Like all KDE and Gnome applications, it works in any desktop environment.
Special mention: K9Copy copies DVDs and compresses them to fit on a standard 4.7 GB blank, so you don't need to spend wads of money on expensive 8.5 GB double-layer blank DVDs.