Ubuntu systems generally run with Ubuntu kernels which are based on mainline kernels and also have numerous patches for bug fixes, security fixes and some extra hardware support. Sometimes it can be useful to run your system with an unmodified mailine kernel, for example, when one needs to check if problems have been fixed with an upstream kernel or if a Ubuntu patch causes a regression.
My colleague Andy Whitcroft (~apw) has worked hard to provide us now with mainline kernel builds. These kernels are unmodified kernel source, built against the Ubuntu kernel configuration files and packaged as .deb files for allow simple installation and testing.
The mainline kernels archive is located at: http://kernel.ubuntu.com/~kernel-ppa/mainline/
and there are a full set of instructions available on the MainlineBuilds wiki page.
Buyer beware. These kernels do not include any of the Ubuntu specific drivers and there are no restricted modules either, so your mileage may vary. Also note that the further the kernel version is from the base kernel the more likely one is to get incompatabilites with user space. Also, the kernel team does not support these kernels, so use them at your own risk.