99 Names of Allah

Allah! there is no god but He! To Him belong the Most Beautiful Names. (Qur'an 20:8)

"He is Allah, the Creator, the Originator, The Fashioner, to Him belong the most beautiful names: whatever is in the heavens and on earth, do declare His praises and glory. And He is the Exalted in Might, The Wise.
(Qur'an 59:24)

"The most beautiful names belong to God: so call on Him by them;..." (Qur'an 7:180)

Abu Huraira reported Allah's Messenger (may peace be upon him) as saying: There are ninety-nine names of Allah; he who commits them to memory would get into Paradise. Verily, Allah is Odd (He is one, and it is an odd number) and He loves odd number. And in the narration of Ibn 'Umar (the words are): "He who enumerated them."
Muslim ibn al-Hajjaj Nishapuri, Sahih Muslim

Allah (subhanahu wa ta`ala)'s names are not limited to 99, which is a common misconception. There are a couple of evidences, one is the du`aa where one calls upon Allah by the names He (subhanahu wa ta`ala) has kept to Himself (obviously not taking these names since Allah has not revealed them to us); another is the fact that in the narrations of the famous ninety nine names hadith that do contain 99 names, the names are not consistent between narrations (for example, imam al-bayhaqi reports two versions of this hadith, with different 99 names in each). It is suggested by one commentator that the names were not explicitly stated by the rasul (sallallahu `alayhi wa sallam).