You can decide whether Open Source applications work for you without installing Linux. Why? Most come in Mac and PC-compatible versions as well. Start by downloading and trying out the big-ticket items - Open Office, Firefox and the Thunderbird e-mail client - on your PC, replacements for your must-use, for-fee applications. Wikipedia.org has a good list of additional software.
If you can handle those file formats, you've made it. Alternately, can you substitute network-based applications for the ones installed on your PC? A warning: If you're heavily reliant on Adobe image-editing software, take a critical look at sGIMP, the most popular Open Source alternative. Of all the common applications, it may be the hardest to get used to.
Next, you can try out various Linux operating systems without installing Linux on your PC. That's because most Linux distributions are available on "live CD" - software you can download, copy to a CD, then run directly from that disk without ever touching your hard drive. If you like it, you can then install it on the hard drive. You can get a comprehensive list of distributions, along with reviews, at distrowatch.com.
I'd suggest you take a look at Ubuntu, Mint and gOS; for older systems there are ultra-compact DSL Linux and Puppy Linux. The live distribution will let you begin to test compatibility with your existing hardware. If you have a fast broadband connection, you should be able to download and look at 4 distros. Netbook buyers should look up the distribution used at the company's Web site. Most times, you should be able to download a free copy as a live CD. Both Ubuntu and gOS offer these packages.
If you have any Windows applications that you want to run, experiment with Wine, which is included with many Linux distributions, or downloaded separately at http://www.winehq.org. Wine is set up to allow developers to create specific compatibility code for individual Windows programs. When it works, it can work well, better than so-called emulation software, and you'll get a list of these apps at the Wine site. But compatibility code isn't always available or up to date.
Linux distributions invariably include a lavish range of software on the installation download. But there's also more stored at so-called depositories - sites that are specific to the version of Linux you are using. There's usually one maintained by the "official" sponsor of the distribution, and another set up by users, so you should also check out the repositories for specific goodies. If a particular program that interests you isn't in the repository, you can still install it. However, you'll have to dig into the guts of Linux to do so, which can be a daunting task.