If you are running on Mac OS X ensure you do not use AppleDouble encoding, or you may give away your username. Otherwise, 7-Zip is safe to use and is the recommend archive format for most purposes.
The Unix tar command by default will include the user and group owning each file which may give away information about you (likely including your name). Using tar to create an archive inside a VM where you run under a username which does not identify you without any extra options is safe. Compressing .tar files using GZIP, bzip2, LZMA or xv will not pose any risk but is not strictly necessary as freenet can compress when inserting.
The basic zip format does not encode anything dangerous, but some software can add additional metadata which may give away information you would prefer not to give away, if the software you are using to create the .zip file supports extra fields it recommended that you disable any fields that may give away information about you.
Audio files will often contain metadata such as ID3, APE and Vorbis comment which are usually used to store song and artist information as well as cover art but if you created the file may contain identifying information, a tag editor should be able to remove them. Many audio files will exist in a container format, which can include other types of file that may also need checking. Some devices produce audio with EXIF metadata which you should remove before inserting (this seems to be only .wav files produced by digital cameras).
Microsoft Office Formats
Are well known for leaking information their authors did not want leaked, better to convert it to another format.
Open Document Formats
LibreOffice (the most common program for creating these files) by default includes the author name in creation and modification records, to prevent your name from ending up in the file Properties you can either not set it in the LibreOffice properties or you can remove it from a file before uploading by bringing up the Properties' dialog box by selecting File then Properties from the menu. Make sure that apply user data is unchecked then press Reset and make sure created has only the date and time and that modified is blank then click OK and save the document.
Some programs which export to pdf will fill in the Author field in the properties with your name when you run them, it would be a good idea to disable that feature or use software which does not automatically fill in the Author field.
Some HTML editors will insert META tags which may give away information about you, if you use a GUI HTML editor you should RTFS at some point in time to see what meta tags if any are inserted. Some HTML editors also have ‘distinctive’ markup which may allow someone to determine what editor you used to create the site.
The only metadata the GIF format supports is a comment, all you need to do is make sure that the comment does not including any identifying details.
The JPEG format supports a comment as well as EXIF and XMP metadata which may give away information about you (as well as the location of a photograph if it was taken with a camera phone). You should remove the metadata before inserting. Also supported are ICC color profiles which may identify you if you have calibrated the source of the image, removing this or setting to sRGB or AdobeRGB is recommended.
The PNG format supports a comment as well as ICC color profiles, all you need to do is make sure that the comment does not including any identifying details and that the color profile is either non-existent or set to sRGB or AdobeRGB.
Video files may contain EXIF or XMP metadata which could identify you. Many video files will exist in a container format which can include other types of file that may also need checking.