Windows only: Update Notifier isn't a new kind of software, but it is a rather straightforward and simple one that seems to Just Work—especially in its support for portable applications.
Update Notifier installs, asks for shortcut placements, then simply runs and checks your installed applications, including any *.paf.exe portable applications, for updates against its server. You can filter which applications it checks for, halt individual programs from being checked, and change what Update Notifier notifies you about—major upgrades, minor bug fixes, and the like. A nice, unique feature of Update Notifier is that it can start up with your system, but you can set a delay on its startup to prevent stalling and slowdown—even though this application doesn't really more than a few megabytes of RAM.
Honestly, there's not a huge difference in the functionality between Update Notifier and something like FileHippo Update Checker —what matters is what applications are covered and recognised. Update Notifier picked up nearly all of my small list of programs on an XP installation, but so does FileHippo. If nothing else, though, I like Update Notifier for its clean interface and streamlined operation.
Update Notifier is a free download for Windows systems only.