al-Mahdi is "the rightly-guided one" who, according to Islamic Hadiths (traditions), will come before the end of time to make the entire world Muslim.  Over the last 1400 years numerous claimants to the mantle of the Mahdi have arisen in both Shi`i and Sunni circles.  Modern belief in the coming of the Mahdi has manifested most famously in the 1979 al-`Utaybi uprising of Sa`udi Arabia, and more recently in the ongoing Mahdist movements (some violent) in Iraq, as well as in the frequently-expressed public prayers of former Iranian President Ahmadinezhad bidding the Mahdi to return and, in the larger Sunni Islamic world, by claims that Usamah bin Ladin might be the (occulted) Mahdi.  Now in 2014 Mahdism is active in Syria, as the jihadist opposition group Jabhat al-Nusra claims to be fighting to prepare the way for his coming; and in the new "Islamic State/caliphate" spanning Syrian and Iraqi territory, as its leadership promotes the upcoming apocalyptic battle with the West at Dabiq, Syria.  This site will track such Mahdi-related movements, aspirations, propaganda and beliefs in both Sunni and Shi`i milieus, as well as other  Muslim eschatological yearnings.
For a primer on Mahdism, see my 2005 article, "What's Worse than Violent Jihadists?," at the History News Network: http://hnn.us/articles/13146.html; for more in-depth info, see the links here to my other writings, including my book on Mahdism.