For pure productivity type work, your G5 is just as solid as other workstations. You have plenty of options to create and edit various word processing, presentation, and spreadsheet files. Let your creative juices flow with tons of slick, sophisticated tools to make beautiful art. Keep in mind that there aren't many options that are up to date. You may have to search eBay, resellers, and the used market to find the software packages you want.
Last Updated: 4/23/2016
iWork '09 is the hands down best productivity suite for your Power Mac G5. It's elegant, gorgeous, bug free, and robust. However, Apple does not sell it anymore, so you may have to dig around with resellers, eBay, and other spots to purchase a copy of yourself. You won't regret it though. Keynote is one of the best presentation apps available. Pages serves a nice hybrid spot between great word processor and capable desktop publisher. Numbers is an effective spreadsheet option. You simply cannot go wrong with iWork.
If you want an older option that might be lying around somewhere, you can get AppleWorks 6, the precursor to the iWorks suite. It still operates quite well in Leopard on PPC machines, if you can find a copy and satisfy yourself with the less Mac OS X-friendly look and feel of the app.
If you like distraction free writing, WriteRoom is a solid, elegant choice. This app really only does RTF files, so there's not a ton of layout type options. When you write though, you have an option to go full screen and hide those emails, messages, and other annoyances grabbing your attention. Version 2.5.1 is the last to run on PowerPC machines.
If you want to stick with Microsoft Office, you'll find two options - Microsoft Office 2004 and 2008. Both are capable suites. Microsoft Office 2004 may run better on slower machines, but it lacks built-in .docx support, for instance. Microsoft Office 2008 ended up dropping VisualBasic & macro support, making it less attractive to some power users. If you want the latest, 2008 is your last option.
Microsoft has some support information about Office 2008 here.
If you need to work with .docx files sent by co-workers, friends, or family members, you've got some options.
If you are looking for MarkDown support for your Power Mac, here are a few options.
If you don't need a full suite or want something a little different, check out some of these options for word processing.
iCloud is Apple's evolving suite of online tools and apps to help make Macs, iOS devices, and Windows machines even more productive. Truly, this set of tools has come a long way since its inception as MobileMe. However, Leopard is incompatible with these tools, for the most part, though there are few workarounds.
iLife '09 is the last version of Apple's content creation suite, including great apps like iMovie HD, GarageBand, iPhoto, and others. It's really excellent software to have and extends the usefulness and possibilities of your Power Mac G5 immensely. The suite is still available from resellers for around $40, maybe less on eBay. iTunes (although you don't need iLife to get it) is still the best option for buying and enjoying great music on your Mac.
BTW, if you need the last version of iTunes for Power Macs, find it here.
Power Mac G5s make capable machines for video editing. There are a variety of video editing options for your G5, from free to expensive, though you will have to search around for older copies to purchase. You can read a roundup of potential options here. Keep in mind that it is hard for me to recommend one video editing application, since most videographers and producers tend to be partial to a particular suite. Use the one that you know best.
Pixelmator is cheaper and just about as good as Adobe Photoshop. I use it regularly. The older PowerPC compatible version is still very capable for editing, mockups, and web design. It's that elegant, responsive, and easy to use. Unfortunately, the last version for PowerPC is 1.5.1, and the app is now well past version 2 for Intel Macs. Unfortunately, I cannot find a download for the PowerPC version on the company website nor do I suspect the company will sell serial numbers for this older version. Stay tuned.
Seashore is an open source Mac native photo editor for Mac OS X Tiger and up based on the technology behind GIMP. It's pretty solid software and looks much better than other open source alternatives. Current version is 0.5.1.
If you want to try GIMP, here's a port for Mac. This has been improving, but at times, the interface has been one of the worst examples of GUI design in the world of software.
The venerable and richly featured Adobe CS packages make excellent content creation tools for your Mac. Once again, you may have to search among resellers and used markets for packages that will work for you. The last version to work on PowerPC Macs is Adobe CS 4.
Preview is built in to Leopard, and it's super fast and awesome. More than just a PDF viewer, Preview lets you rearrange PDFs, modify images, fill in some forms, and export files to different formats. I use it regularly to grab icons from apps for this website. Very useful.
Adobe Reader (version 9.4) is the latest PDF reader straight from the source. For more complex documents, including forms and what not, you may find Reader is more compatible than Preview. However, Adobe software has been notorious lately for security holes and bugs. Use if you need, but Preview is really better in almost every respect.
Audacity is an open source, rock solid audio recorder and editor for Mac. The current version still works on PowerPC machines. You don't really need anything more than this great app to capture audio, modify it, save it in various formats, and enjoy life doing podcasting or basic recording. Go for it.
Amadeus Pro is another solid option for recording and manipulating audio files. It bills itself as a multitrack audio recorder. Very robust and very solid. I use version 1.5.5, but it appears version 2 is still compatible with PowerPC Macs.