Tools and Libraries
These tools will help with image and music conversion and others are useful time savers and some are full blown development kits.
These tools run in your web browser so there is nothing to install and most are drag-and-drop.
Libraries and SDKs
8bit-Unity is a cross-platform C language SDK for 8bit computers (including Atari Lynx). It features an integrated asset pipeline which makes it easy to produce bitmaps, sprites, and music for your game.