Follow this link to get an overview on different JPEG standards.
There is a JPEG-1 decoder I have written from scratch for the ITU T.81 Standard (see JPEG81). It implements most of the Standard except the Hierarchical process:
Originally made for my homebrew OS to display small JPEG coded images. It's stand-alone, has it's own IDCT routine, a scaled integer version of the modified LLM transform.
This JPEG decoder (jpeg81.c) can be found in the image decoder library, RAINBOW on BitBucket.