I have written in the past about the reliability and stability of Linux, and pointed out that it does not have to be rebooted unless there is a kernel upgrade being done. All other pieces of software can be upgraded on the fly. This is in drastic contrast to Windows which needs to be rebooted for a majority of its patches, Linux can run for a very long time without a reboot (we are talking years here).
Ksplice came up with a technology that can patch or install a new kernel on the fly. This takes the minimal reboots of Linux to zero reboots. Ideally, this allows a machine running Linux to run indefinitely.
Now, Ksplice is being offered for free to users of Fedora, the free Linux distribution supported by Red Hat. There are future plans to have it integrated into the Fedora distribution as well. I'm not sure if this is a limited time offer or permanent for Fedora. I'm guessing it will be around for a while though, as Fedora is technically Red Hat's bleeding edge distribution that is released for free and allows software to be refined for Red Hat Enterprise.
Ksplice is of course available for other common distributions like Red Hat Enterprise, Ubuntu, Debian, CentOS, and others at a subscription of $3.95 per month.