This is kept up-to-date in mainline, so get the kernel of your choice from kernel.org and build UML from it.

This tarball contains uml_net, uml_mconsole, uml_switch, tunctl, and a number of other useful utilities. Go here for details on building and installing them.

These are host patches which UML uses to run noticably faster. SKAS3 has been around for years, is widely used, and is considered stable. The latest patch, for 2.6.23, is here. All versions of UML will detect the SKAS3 patch on the host and use it if it is present.

SKAS4 exists because the SKAS3 patch can't be merged to mainline because the interfaces it implements are wrong. SKAS4 implements the same functionality under a much nicer interface.

The current SKAS4 patch, against 2.6.24, is here. This is experimental. In order to use it, you should build both the host and UML from the same tree. Full details are also available at the link above.