I've spent much of the last year working on version 3 of SCI Companion, which lets you create (and mod existing) games that run on Sierra's SCI engine from the late 80s and early 90s (game's such as King's Quest or Leisure Suit Larry).
I released version 2 many years ago, which provided support for EGA SCI0 games. Version 3 now supports VGA (256-color) SCI1 games, which also include digital audio/speech. It comes with a complete template game to get you started, extensive documentation, and there is a small but passionate community to help you out and inspire you.
I have also implemented a decompiler, which can generate source code from the original games script resources, letting you make script patches to old games (or reveal Easter eggs).
Of course, it's all free too!
Here's the official blurb from my site, which I linked above:
SCI Companion is a completely free IDE (integrated development environment) that lets you create games that run on Sierra’s SCI engine. SCI Companion 3 is now available, which offers support for SCI1.1+, which includes VGA graphics and digital audio.
Create 16-color EGA graphical adventures with a text parser interface (in the style of King’s Quest 4, Space Quest 3 or Leisure Suit Larry 3).
Create 256-color VGA graphical adventures with a point-and-click interface (in the style of Space Quest 5, Gabriel Knight, or Quest For Glory 3).
Comes with a complete well-documented class system and template game.
Lets you browse, extract or modify graphics, text and sounds from nearly any SCI game that you own.
Decompile the code behind these classic games to see how they work (or find Easter eggs). Scripting in SCI is accomplished via a language similar to LISP and SmallTalk.
Includes streamlined editors for graphic assets.
And other game resources like text and audio.