The writing systems listed below have yet to be deciphered or have only been partially deciphered. In some cases the writing systems have been deciphered but the languages they were used to write remain a mystery.
A collection of symbols found on many of the artefacts dating from
between 6,000 to 4,500 BC excavated from sites in south-east Europe,
in particular from Vinča near Belgrade. There is no agreement
on whether these symbols are a writing system.
A collection of symbols used in the Indus valley of India between about 3,500 and 2,000 BC. Some believe that these symbols are non-linguistic, while others argue that they represent a Dravidian language.
A script which first appeared in about 2900 BC in Suse (Susa), the
capital of Elam, in south-western Persia (modern Iran). It has yet to
be deciphered and the language it represents in unknown.
A partially deciphered syllabic script used between about 2250 and 2220 BC
in the kingdom of Suse in south-western Persia (modern Iran). It was
named after Elam, the capital of Suse.
The Phaistos Disk was found in the Minoan Palace of Phaistos on Crete in 1908 and is thought to date from the 17th century BC. On it is inscribed an unknown script and there are many theories about the language it represeents and what it means. No other evidence of this script has been found.
Further details: http://en.wikipedia.org/wiki/Phaistos_Disc
The Voynich Manuscript is named after Wilfrid M. Voynich, an antiquarian book dealer who acquired it in 1912. It is lavishly illustrated manuscript codex of 234 pages, written in an unknown script. One theory is that is was written sometime during the 13th century by a Franciscan friar, Roger Bacon (1214-1294). Many attempts have been made to decipher the text but none have succeeded. Some think the manuscript is gibberish, and was probably a practical joke played on Rudolph II.
Further details: http://www.voynich.nu
The Rohonc Codex (Rohonci kódex) is named after the city of Rohonc, in Western Hungary (now Rechnitz, Austria), where it was kept until 1907, when it was moved to Budapest. The origin of the codex is uncertain. In 1838 it was donated to the Hungarian Science Academy by Gusztav Batthyány, a Hungarian count, together with his entire library. It is written in an unknown language and script and has defied all attempts to decipher it.
A script once used on Easter Island until the 1860s, after which knowledge of
the script was lost. The language it represents is Rapa Nui, the Polynesian
language spoken on Easter Island.