Ksplice

[jerenept]

allows you to do kernel updates without restarting.

http://www.ksplice.com/

[CharlesA]

It's only free for Ubuntu desktop, you need to pay to use it with Server.

Why you would run it on a desktop machine, I don't know. :-)

[szymon_g]

it's on GPL2, so you can use it (i.e. it's source, you can compile it by yourself) without paying anything. and it's pretty useless on desktop :)