What is the most heat-tolerant organism?
Methanopyrus kandleri has been able to grow and proliferate at temperatures of up to 122°C when under around 20atm (Takai et. al. 2008).  This was achieved in the lab but the theory should hold true for deep sea thermal vents.  Another strain of M. kandleri survived incubation to 130°C however was unable to reproduce at this temperature. 

One of the key adaptions for thermophiles is the selection towards an extremely thermo-stable copy of the enzyme DNA polymerase.  This allows DNA replication at high temperatures and is similar to the 'Taq' polymerase used in the polymer chain reaction in labs, derived from another thermophile, Thermus aquaticus.    



References


 Takai K, Nakamura K, Toki T, Tsunogai U, Miyazaki M, Miyazaki J, Hirayama H, Nakagawa S, Nunoura T, Horikoshi K. 2008. Cell Proliferation at 122°C and Isotopically Heavy CH4 Production by a Hyperthermophilic Methanogen Under High-Pressure Cultivation. PNAS 105: 10949–10954.