Some PDF files downloaded from the Internet may be protected with password, most of which are "Owner Password Protection" type (used to prevent copying, editing, and printing the PDF file), a few of them will be "User Password Protection" type (also referred to as "Open Password Protection", others can not open it without the correct password). As to the former case, we can use all kind of PDF password removers to remove the passwords directly with ease; but for the latter, the user password is required, or you have to make brute force attacks.

On hard disk, when you delete some folders, files or even an entire partition, all data is still existing in the same place where it was, and will always be there until overwritten. In fact, the deleting operation is just delete the pointer to the data on disk from MFT (Master File Table), so as to allow the operating system to perform writing operation there. Any data recovery software use this principle to work. Of course, you can delete the data truly and completely. But that requires you to ask special data eraser for help.

Making multiple small images spliced into a large one (sprites sheet), for game development, this can save the storage space and loaded memory that texture images require in game; for web design, this (CSS sprites) can greatly reduce the images loading frequency and time, so as to make web pages load faster. If you want to use Photoshop to do this, that's too stupid and inefficient. Or if you gave Zwoptex up, then it's time to turn to this TexturePacker from Germany.

ACDSee (now called: ACDSee Photo Studio) was ever one of the long-standing (first released in 1994), most popular image browsing software that came with friendly UI, simple & humanized operation, high-quality and very fast image decoding method, comprehensive image formats support and powerful graphics file managing functions. All this won and accumulated the most users around the world for it (it basically was must-have especially for architecture and image processing related industries, including photography enthusiasts and professional graphic designers.

Unity (AKA: Unity3D/Unity2D, U3D for short) is currently the most popular multi-platform free game engine developed by Unity Technologies from Denmark (mainly written in C, C++ and C#, and first released in 2005). Users with C# and JavaScript programming experience can get started more quickly.