What type of a design/design philosophy/gesture can be a good alternative to tabs in an iphone or an android application? Tabs are good and tell the user that there is another category/page/functionality in the app, but they tend to take up some space, and this can be a disadvantage on say low end android phones with small screen sizes.
A zooming user interface (ZUI) might be what you are looking for.
Pinch-to-zoom is a well established and frequently used gesture on multi-touch mobile devices. Zooming out would show the various available pages; you would then zoom in for a more detailed view of a particular item.
As far as I could see from the common trend, a few apps are moving the menu in one side of the phone, like some famous app do (Path, Facebok ...). The result would be:
The user has to swipe the navigation bar or touch the menu button, and then the menu will be revealed. This allows to save some space. Moreover, considering that now this paradigm is used by many well known apps, it's easier for the user to understand.