Simple Grid is described as 'CSS grid for your website' and is an app in the Development category. There are more than 50 alternatives to Simple Grid for a variety of platforms, including Online / Web-based, Self-Hosted solutions, Mac, Windows and Linux. The best alternative is Bootstrap, which is both free and Open Source. Other great apps like Simple Grid are Bulma (Free, Open Source), Materialize (Free, Open Source), UIkit (Free, Open Source) and Semantic UI (Free, Open Source).
Juiced has many of the same layout features that you know and love, but improves upon them with better mobile targeting, custom column ordering, better alignment options, understandable HTML, and less boilerplate styles.
Flat UI Pro is made on the basis of Twitter Bootstrap in a stunning flat-style, and the kit also includes a PSD version for designers. Flat UI Pro contains a huge number of basic components, icons and glyphs.
The 960 Grid System is an effort to streamline web development workflow by providing commonly used dimensions, based on a width of 960 pixels. There are two variants: 12 and 16 columns, which can be used separately or in tandem.