What are the boiling points of DMSO-water concentrations?
The binary mixture of water and DMSO do not form an azeotrope (a constant boiling mixture, where the vapour produced has fixed percentages of each component). Thus, water will boil from the solution first at a bit above 100 ºC, followed by the boiling of DMSO at around 189 ºC. 

The reason for the drastic change in freezing point is a little more complicated. Freezing point depression is a common phenomenon; for example, it's the reason sea water freezes at a lower temperature than pure water. In these cases, the reason for the freezing point depression are typically entropic.

However, in the case of DMSO and water, the depression is of particular note—mixtures can remain liquid down to -140 ºC, compared to the normal freezing points of 0 ºC and 19 ºC for water and DMSO, respectively. Quantum mechanical studies have suggested that the formation of DMSO-water clusters is at least partially responsible for the massive freezing point depression. These clusters are strongly hydrogen-bonded, but interact far less strongly with other clusters, making it more difficult to form a solid lattice (and thus a solid).



Kirchner, B.; Reiher, M. J. Am. Chem. Soc., 2002, 124 (21), pp 6206–6215