Mile
Zero

How many "pro" musicians are using Garageband as a serious music production tool? Peter at CDM sorts them into a few basic categories in a post on improvements in Garageband '08, which finally includes some features that I would say a real DAW needs to have: plugin automation, multiple time signatures, and multi-take recording. It's still a toy, but it's becoming a decent entry-level recording tool, instead of just another ACID clone.

It's been said that Microsoft should think about offering a comparable production program for Windows, so PC users can also have a (sort-of) free music creation tool. But I'm not so sure that we need it. I'm not terribly conversant with the state of OS X freeware, but there's lots of free software on the PC that offers the same functionality (or more) as Garageband, and after three years of experimentation on the cheap, I think I've probably used most of it. Here's a few good options for newcomers to audio production on Windows:

• Cubase LE - No, it's not technically free, since it only comes with OEM hardware. But look at it this way: on any computer platform, if you're doing more than just dragging loops onto a timeline (and please: we have too much of that already), you're going to need to buy an external interface anyway, because built-in sound cards are really bad input devices. There's a lot of great hardware that comes with Cubase LE, including stuff from Tascam, Lexicon, and Presonus. And this is, in my opinion, as close as you can get to pro software at this price level. It's got full automation, lots of tracks, records up to 8 inputs at a time, uses regular VST plugins, and includes a decent set of built-in effects. Honestly, there's not much missing here.
• Krystal - Krystal is free from Kreatives, and has a lot to recommend it. It's a multitrack recording program with an attractive and straightforward interface and VST support (with four plugins included). It's roughly comparable to Garageband 2, but doesn't have the looping support. It also lacks MIDI functionality, plugin automation, and more complicated routing options, and the input routing is a little weird. But for all that, again, you can do pretty well with Krystal if your goal is simply to record and mix audio. I wouldn't hesitate to use it if I needed to track a band at the last minute, especially coupled with a few tools (freeware drum machines, for example) to supplement its functionality. There's supposed to be an updated version (K2) coming soon, but they've been saying that for a couple of years now.
• Reaper - Like Cubase LE, Reaper isn't technically free. It's supposed to be $40 for personal use, and $200 for commercial purposes. But you can download the shareware version, which has full functionality and never expires--Justin Frankel, the head programmer, is the guy behind WinAmp and Gnutella, and he's not a real stickler about licensing. The good news is that Reaper does everything that Sonar, Cubase, and Pro Tools do, and some things that they don't. Reaper has unbelievably flexible routing options and prides itself on its ability to change almost anything while recording. It shows up as an ASIO sound card for recording from other applications, or it supports ReWire if you want. It's gotten rave reviews from music mags. The installer is only two megs. The only bad news, as far as I'm concerned, is that some parts of Reaper still look and feel a little clumsy. It's skinnable, but that doesn't necessarily make it easier to use. Still, usability has improved immeasurably since I first used it (version .62, I think), so it'll probably only get better. This is an insanely powerful tool, and there's nothing on Mac or PC that competes with it in terms of value.
• Audacity - I include Audacity as a word of warning only. I actually don't recommend using it for recording projects, for a variety of reasons. It doesn't have much in the way of plugin support, it only records in stereo, there's no support for low-latency recording like ASIO, and it distorts at the drop of a hat (I'm guessing it actually mixes at the same bit depth as the project). Audacity is one of those things that people recommend all the time, but really shouldn't. It teaches a lot of bad habits, and there are much better options out there for free.
• Ableton Live Lite - Ableton Live (the full version) is a real paradigm shifter for audio software. Live Lite, the OEM version that comes with just about everything M-Audio makes, lets you see all the great stuff from the full version, but locks a lot of it away from actual use. This is frustrating. The good news is that it's practically free, much like Cubase LE. Buy a MIDI keyboard, and you'll probably get Live Lite. Buy any of Line 6's computer products, and you'll get Live Lite. I've owned four or five of these CDs, at least, and had to give them away to other people--you could probably ask around and get a copy, if you have musician friends. For free, Live Lite's not a bad deal--it's basically a 23rd century four-track tape machine. Ableton just released Live LE to try to cater to the beginner/low-cost market, but it's crippled down to practically the same level. If you're curious about Live, start with Lite and upgrade, instead of LE.

There's also a whole world of standalone, budget (~$100) software "for beginners" out there, but I tend to think that it's kind of a waste, depending on the package. Midrange copies of Sonar or Pro Tools are just not that much more money than something like Sequel or Project 5, and the capabilities they offer go much farther. Even the low-end versions, like Sonar Home Studio, can be pretty good, and the upgrade path is easier on the wallet.