Why is the boiling point of pentane (36.1 °C) slightly higher than that of diethyl ether (34.6 °C)?
This is perhaps not a very scientific answer, but consider how branched alkanes have a lower boiling point than straight chain alkanes.  This is due to the inability of the individual molecules to "stack" (if you want to get technical it's to do with surface area).  Now imagine stacking diethyl ether molecules.  The problem arises that two oxygen molecules (2 lone pairs each) on two separate diethyl ether molecules would not want to stack directly on top of eachother as the oxygen atoms have decently high electron densities.  The repulsive force would likely be minimal, as demonstrated by the insignificant difference in boiling points between diethyl ether and n-pentane.  However, this is only an educated guess.

To strengthen my argument, check out Diethyl sulfide, which has a much higher b.p. than both n-pentane and diethyl ether.  This could be due to the larger atomic radius (and therefore larger surface area) of Sulfur in comparison to Oxygen.  The charge has a larger area to be distributed over, resulting in less repulsion between "stacked" sulfur atoms.