With Snow Leopard, the next version of Mac OS X,
expected rumored to ship out any week day now, it's worth looking at one thing that can make upgrading to 10.6 a lot easier (at least on an Intel Mac-as most PowerPC Mac users should know by now, Leopard 10.5 is the end of the road for you: Snow Leopard is Intel-only).
Snow Leopard is mostly about optimizing Leopard's performance rather than introducing new features. Part of that optimization is that OS X 10.6 is expected to save users several gigabytes of space on their hard drives versus a 10.5 installation. And part of the reason that Snow Leopard is able to pare down that much space is that Rosetta is now an optional installation.
(Update: as many commenters have noted, the Rosetta program itself doesn't take up much space - only a couple of megabytes. Most of the space savings in Snow Leopard is from Apple stripping out PowerPC binaries from the apps and OS libraries. It's still worth going through your apps and updating the PowerPC-only apps to universal binaries, however, because you will still see a significant boost in performance by doing so.)
Introduced in 2005, not long after Apple announced its transition to Intel processors, Rosetta is a dynamic translator that allows legacy PowerPC applications to run on Intel processors. It was intended as a stopgap measure to allow PowerPC applications to continue to run on Intel-powered Macs until developers were able to update their applications to support either universal binaries or Intel-only code.
Applications which run under Rosetta provide slower performance than their universal binary counterparts because the CPU has to translate Intel instructions into PowerPC, so developers definitely had an incentive to switch to universal binaries. With four years having passed since the Intel transition, almost all applications for the Mac now run under a universal binary, which makes Rosetta largely unnecessary-hence its inclusion in OS X Snow Leopard as an optional installation.
So, why not save some space on your hard drive and leave Rosetta out? Well, if you do that, any applications you have that still have PowerPC only code won't run at all. (Update: Apparently Rosetta will download on demand if you try to run a PowerPC-only application.) Rosetta is absolutely necessary to run those applications. But, before Snow Leopard drops to consumers, you can take one simple step that will save you a lot of trouble.