I will discuss Sirius and three other nearby visual binaries in which one component is a white dwarf (WD). Our Hubble Space Telescope astrometry of Sirius and Procyon obtained over the past 2 decades, combined with ground-based data going back to the 19th century, provides precise dynamical masses for the components. A recent analysis of historical and new ground-based data (and one HST point) for the 40 Eridani BC system has also updated the mass of the iconic WD 40 Eri B, the first one to be discovered. For Stein 2051B, we determined its mass through HST astrometry of a background star, relativistically deflected as the WD passed closely in front. Theoretical WD mass-radius relations agree very well with the new data for all 4 WDs. The derived age (cooling time plus main-sequence evolution) of Sirius B agrees well with that of the primary Sirius A. However, several astrophysical puzzles remain, including the high orbital eccentricities of the binaries, the lack of direct evidence for the expected past interactions between the stars, and the evidence that many WDs have very thin outer hydrogen layers compared to theoretical expectation.