Msieve 1.5.3 for Mac was available to download from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The program belongs to Developer Tools. Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. This free software for Mac OS X is an intellectual property of Jasonp Sf.
From the developer:
Msieve is an open-source C library implementing a suite of algorithms to factor large integers. The application contains an implementation of the SIQS and GNFS algorithms; the latter has helped complete some of the largest public factorizations known.
You may want to check out more Mac applications, such as AcidSearch Installer, Browsealoud or RAGE WebDesign, which might be similar to Msieve.