Go to Safed. No really, fly to Tel Aviv, go to Tachana Hamerkazit, and take an Egged bus to Safed. It's beautiful, a city in the air, the home of artists and mystics, the birthplace of Lurianic Kaballa. The city is cold, for Israel, but the views are incredible. And there is Hebrew graffiti everywhere, all the same:

Na Nach Nachma Nachman Me'Uman
נ נח נחמ נחמן מאומן

The phrase translates quite simply as Na Nach Nachma Nachman from Uman, as well as, due to a quirk of homonymity, Na Nach Nachma Nachman is faithful.

The graffiti is courtesy of the aptly named Nanach sect, founded during the 20th century by Rabbi Yisroel Ber Odesser of Tiberias. Rabbi Odesser was born in 1888 to a family of Karliner Hasidim, but as a young man he came across the book Histopchus HaNefesh (Outpouring of the Soul), by Rabbi Nachman of Breslav, the founder of the Breslover Hasidic sect, who had died in 1810 and is buried in the town of Uman, Ukraine. According to legend, Odesser found the book in a trash can. As a result of reading Histopchus HaNefesh, Rabbi Odesser became a Breslover Hasid. Please recall that Breslover Hasidim are wont to be a little bit loony

According to Rabbi Odesser's own testimony, in 1922, on the fast day of the 17th of Tamuz, he succumbed to temptation and ate some food. Immediately thereafter he fell into a deep dejection and penitence. Later that day a powerful impluse came upon him, urging him to leave the synogogue, go to his room, and take a book from the bookcase. Inside the covers of book Rabbi Odesser found a petek, that is to say a short letter, written in Hebrew and Yiddish. The text of the letter follows:

It was very hard for me to come down to you, my dear student, and to tell you that I have recieved great pleasure from your worship, and to you I have said, "My fire shall burn until the coming of the Messiah." Therefore be strong and have courage in your worship. NA NACH NACHMA NACHMAN ME'UMAN. And with this I will reveal to you a secret, and it is: Be full and pile up from one extremity to the other. Patzpatzyah. And with strong worship you'll bring him. And my sign to you is that on the 17th of Tamuz they will say that you are not fasting.¹

In fact, it was the 17th of Tamuz, and Rabbi Odesser had only recently violated the fast. Odesser took the phrase Na Nach Nachma Nachman Me'Uman to be the signature on the letter, and he termed it the "New Song". His disciples offer up the New Song as a prayer to Rav Nachman himself, beseeching him to alleviate their suffering in general and to hasten the arrival of the Messiah. It is a tenet of Nanachian belief that the arrival of the Messiah will only be hastened if a critical number of Jews recite the New Song. Nanachim therefore graffiti the prayer everywhere that they go, often in blue letters. Even more common are yellow bumper stickers on which the phrase in printed in black; these are stuck to cars, street signs, railings, etc. The intention of the Nanachim is that passers-by reading the grafitti will be inadvertantly praying, at least mentally, and adding their voices to the existing clamor.

Mainstream Breslover Hasidim do everything they can to distance themselves from the Nanachim, whose tenets also include the offensive demand is that the body of Rav Nachman be removed from its tomb in Uman and reinterred in Israel. As it is, the Breslover Hasidism have a long-standing tradition to make a yearly pilgrimage to Uman for the holiday of Rosh Hashana.

Yes I did just use the term "mainstream Breslover Hasidim".
¹From http://www.moharan.com, which appears to be at least an unofficial website of the Nanach sect. That website includes the text of the petek and quotes Rabbi Odesser's story in his own words. The translation from Hebrew is my own. Patzpatzya sounds like the name of an angel but I'm not sure.