iPhone developer Ripdev has released desktop software for Mac (PC soon) that can push applications to your iPhone without jailbreaking. InstallerApp
will not install a set of command-line utilities and unlock the system partition as usually is done by some popular jailbreak solutions.
As Engadget points out, technically it’s still jailbreaking your iPhone, but considering that you’re not messing with the system software, this is a more stable approach to getting non-Apple-approved apps on your iPhone than other methods.
The app lets you browse non-iTunes Store application repositories and choose packages to install. The interface is easy to use, but unfortunately many of the apps are only briefly described, and only some have “more info” links. You’ll want to do some research before choosing which apps to install.