You can rearrange the resistors like this to help visualize how to simplify:
simulate this circuit – Schematic created using CircuitLab
Tyler's answer explains how to find the thevenin resistance. To then find the thevenin voltage, it's the open-circuit voltage between your terminals a and b. (source: http://hyperphysics.phy-astr.gsu.edu/hbase/electric/thevenin.html#c3), which turns out to be EDIT: 1.25V (yes, as Mario points out, the circuit simulator is correct).
Voltage divider: top resistance is R2, bottom resistance (across a and b) is R13 (R4 does not come into play in the voltage divider because we're looking for the voltage across and b terminals. Think about it from the bottom-up, from the zero volt terminal up). So:
1 / R13 = 1/R1 + 1/R3
R13 = 1/(1/5 + 1/2.5) = 5/3 Ω
Vth = 5V * (R13 / (R13+R2))
Vth = 5V * (5/3Ω / (5/3Ω + 5Ω)) = 1.25V