Official (stable) builds
Official stable builds of ONScripter-EN can be found on the Releases page of Uncle Mion's ONScripter Corner. I used to maintain my own page with Mac OS X versions when things were less stable and I was doing builds manually on my own machine, but since things have relatively stabilized and Mion has been doing official releases, that's the best place to get the official builds.
Unstable "nightly" builds
We now use a semi-automated build system for ONScripter-EN called ONSBuild. When we invoke ONSBuild, it grabs the latest version of the code from the Subversion repository, builds it for OS X and Linux, and uploads the resulting builds to my web server. If you want to try the latest and greatest version and there's no official release, you can check the ONSBuild output directory to see if there's a build there. Details about each build can usually be found in the CHANGES file.
ONSBuild only makes Mac OS X and Linux (both i386 and x86_64) builds; I don't have it set up to do Windows at the moment.