In 2003, Apple released the last generation of G3 iBook. These machines featured processors as fast as 900MHz, ATI Radeon 7500 video cards and the last of Apple's 802.11b AirPort cards. I am the proud owner of one such machine and in my opinion, it makes the best Free Desktop machine that I have ever used.
The G3 processor, while a slouch for 2006 standards of 90C dual-core wonders, is quite usable for browsing, mail and web development. Moreover, the G3 never runs too hot and the battery life is outstanding.
The AirPort card, based on a prism2 chipset, has been widely supported in Linux for years, as has the ATI Radeon 700, which includes Open Source DRI drivers, offering light hardware accelerated 3D graphics.
Perhaps most importantly, the iBook uses Apple's PMU rather than the troublesome APM or ACPI. This alone makes the iBook particularly well suited as a Linux laptop, as it means that power management, such as dimming the screen, suspend to RAM and other troublesome power-related issues do simply work perfectly, as expected.
Of course, what sets this machine apart as a great Free Desktop machine, rather than "just" a good Linux laptop is once again the G3 processor. This means that this isn't just another x86 box. Linux on x86 has many closed-source benefits that simply don't exist on other platforms such as PowerPC. Flash, decent Java, VMWare and media codecs that use Windows DLLs will not work on the PowerPC. This means that, with the exception of Opera, if it's not Open Source, you will have to do without.
Whether or not this would work for you as a main machine is really down to the individual. I'll admit that I miss not having YouTube. On the other hand, this completely Open Source desktop is still remarkably viable. It even helps illustrate some of the important Open Source replacement projects underway that are often ignored by us regular users in the x86 world. For instance, gplflash is a GPL Flash implementation that, while not yet up to the job of running all Flash 7 programs, is able to run the popular Homestar Runner tunes. There's also gcj and the GNU ClassPath project that provide GPL implementations of Java that are sufficiently complete to run high-profile programs such as Eclipse and OpenOffice's BASE.
A G3 iBook can be found on eBay these days for around $300. If you're looking for a good, small Linux laptop and either only need or only want to run Free applications, I believe that even in October of 2006, the iBook is one of the best choices out there.