TexturePacker - Texture Packer is hands down the best sprite sheet generation tool on the market.
Beeceptor - Unblock yourself from API dependencies, and build & integrate with APIs fast. Beeceptor helps you build a mock Rest API in a few seconds.
Free Texture Packer - Free Texture Packer is an open source tool that allows you to pack multiple images into one atlas for you games or sites.
Webhook.site - Instantly generate a free, unique URL and email address to test, inspect, and automate (with a visual workflow editor and scripts) incoming HTTP requests and emails.
Sprite Monkey - Small and fast but still full-featured sprite sheet editor.
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.