Outguess is a steganography tools that allows a user to embedd hidden data inside of a JPEG-image file (and some other image formats). It stores the data by modifying redundant information, so that it doesn't cause visual changes to the image. Outguess became widely known since Cicada 3301 posted their first image on 4chan in 2012. The image contained a hidden message that lead the reader to their recruitment tests.
OpenPuff is a steganography tool for Microsoft Windows that supports a wide range of formats (images, audios, videos) for embedding hidden data. OpenPuff is partially open source.
MP3stego is a steganography tool for Microsoft Windows that hides data in MP3-files. It hides data in the MP3-files during the compression process. MP3stego is open source.
Steghide is a steganography tool for Linux and Microsoft Windows that supports JPEG, BMP, WAV and AU files. Steghide is open source.
The Baconian cipher, or Bacon's cipher, is a method for steganography invented by Francis Bacon in 1605. The writer must make use of two different typefaces for this steganography method. The message is converted to a group of five "code letters" for each letter in the message. A false message is then written, where each letter is presented by the appropriate typeface for the corresponding code letter. Instead of using two typefaces, any other method can be used that allows two distinct representations for each character.
Spectrograms is a method of hiding images inside of an audio or video file. Basically the y-scale is represented by frequencies, while the x-scale represents time. The result is weird sounds and strange sound patterns. For example the viral 11B-X-1371 video used this method to embed creepy images. To view spectrograms, you can use a tool such as Audacity and there switch the visualisation from Waveform to Spectrogram. If you are analysing a video file, you need to extract the audio before opening it in Audacity.
Sometimes, clues can be presented as music. There are some tools available that can listen to any music through a microphone or a audio file, and try to identify it for you. Shazam is one of the most popular tools.
Image information (Exif)
Some image file formats, such as JPG, store various metadata about their contents. This can be information such as where a photo was taken or which camera model was used. This can be useful information for puzzle solving, especially for geocaching or puzzles that involve coordinates.
Image manipulation and editors
A very basic and classic method to hide a message in an image is to write it with nearly the same color as the background. In that way it will be invisible to the eye, unless you for example fill the background with another color. Othen hiding methods are more subtile and require more advanced methods to find the hidden message. A good image editor is a must for a puzzle solver.
Single-image stereograms, or autostereograms, are images that when viewed correctly gives the perception of 3D depth. Viewing them requires some training, as our eyes are not used to converging/diverging our focus. Tools can therefore be helpful for those who have difficulties seeing the 3D shapes.