Since 02.01.2012 a new regulation of residence for foreigners is in force. Under this new rule, tourists may stay visa-free in Turkey 90 days (beginning on the date of first entry). But the total stay within 180 days cannot exceed 90 days. Shorter stays that are falling into this period are added.
For example, you enter Turkey on 02/01/2012, this day the 180 day period starts. Within these 180 days you are allowed to spend 90 days in Turkey. Either you can spend these 90 days in a row in Turkey or choose shorter stays. That means you can enter and leave Turkey several times as long as the accumulated total stay does not exceed 90 days.
This regulation meets the requirements for obtaining a "Schengen visa visitor" in the Schengen countries within the European Union. If you want to stay longer than 90 days in Turkey, you are advised to apply for a residence permit. Especially the employment of illegal workers from Eastern European States should be avoided under this new law.