Keeping software applications up-to-date on personal computers has always been a sore point. The best solution I have used on any platform is apt-get or aptitude for Debian-based linux distributions (which includes Ubuntu).
On the Mac, the best solution I have seen is AppFresh. It is still in beta, but it is already very useful. It scans all of the applications installed on my machine and with one click I can automatically install application updates.