Open Source or Indie Tower Games

There have been several attempts at making new games that were supposed to offer similar gameplay as SimTower/YootTower with the goal of eventually becoming better. So far, only 3 of note are playable. They are listed in order of creation.

Highrise Developer
Highrise Developer was designed as an HD Open-Sourch SimTower clone. It uses C++ for the programming language and OpenGL for rendering. The developers of the project made it more as a proof-of-concept and a practice/exercise of their C++ skills. It was planned after the original OpenTower program stopped working in 2008 because of depreciated code in SFML that required a very early version to run without errors. It made it to a 3rd Alpha version before the programmers became too busy in real life to continue programming in 2009. Highrise Developer has zooming and can support multiples of 36 (24px for floor, 12px for space between floors) pixels. The majority of the graphics pack is based off AzemOcram's earlier graphics art with various improvements and additions, including animations (AzemOcram has not made any animations until 2013, and even that is basic).

This is available on SourceForge.

OpenSkyscraper
OpenSkyscraper was designed as a 2D SimTower clone that extracts SimTower's graphics from an installed copy of the game. Most of its programming was halted after March of 2013 though some minor bug fixes and proposals have been committed on GitHub. The developers planned to enable an in-house as well as custom graphics packs and sound packs after they fixed up the AI.

IsoTower
IsoTower is the first isometric (prerendered) 3D tower simulator and it is not open source but an inexpensive (but not free) Indie game. It has its own website and is undergoing continuous improvements though the next release is being thoroughly tested for bugs. Purchasing has been halted since November 2013 as of February 2014 though free demos are available. Development has continued through the present (2015) and is likely to have a release (that is sold) by the end of the year.

Project Highrise
Project Highrise is being developed by somasim, the creators of 1849. This will most likely be the highest quality tower simulator game on the market. They are going after a mid-20th century style that is still cut-away 2D front view like SimTower. Their game is planned on having the most complex simulation of any tower simulator to date. Their focus is on building a successful high rise and managing the residential and businesses and not on micromanaging individual people.