Paul I is a direct ancestor of King Willem-Alexander through two of his daughters Elena Pavlovna married the hereditary Grand Duke of Mecklenburg-Schwerin and Anna Pavlovna married the later king Willem II of the Netherlands.
Through Elena he is also the ancestor of Margrethe II of Denmark (a granddaughter of Alexandrine of Mecklenburg-Schwerin with both parents descending from Paul I).
King Felipe VI of Spain is another reigning descendant. His mother Sofia descents from Queen Olga Constantinovna of Russia another descendant of Paul I.
As do the Duke of Kent and his siblings through princess Marina.
King Willem I and his first cousin Friedrich Wihelm III of Prussia have a lot of reigning descendants as well.
Their granddaughter Louise of Orange-Nassau married Karl XV of Sweden. Their marriage produced two children, a son who died young and a daughter Louisa Bernadotte.
From her marriage to Frederik of Denmark she is the ancestor of the Queen of Denmark, the King of Norway, the King of the Belgians and the Grand Duke of Luxembourg.
Willem I and Friedrich Wilhelm III are both also direct ancestors of king Willem Alexander of the Netherlands, be it through a different child.
Friedrich Wilhelm III can also add Felipe VI of Spain to his descendants as his mother descents from Sophie of Prussia.
I believe he has six direct descendants living today, who are reigning monarchs in Europe -- namely, the sovereigns of Norway, Sweden, Denmark, Belgium, Luxembourg, and Liechtenstein. That's actually the same number claimed by King Christian IX of Denmark (the so-called "Father-in-Law of Europe"), and one more than Queen Victoria of Great Britain.
What I'm interested in are additional examples of past royals (within the last two centuries) who belonged to dynasties no longer reigning today, who nevertheless have sovereign descendants. Is it true that only two sovereigns today (Spain and Liechtenstein) are descended from the Habsburgs?
The five Protestant kings are out; the House of Saxe-Coburg and Gotha in Belgium war originally Protestant (and King Leopold II had no surviving son by Archduchess Marie Henriette). The House of Nassau-Weilburg in Luxembourg was also originally Protestant. And the Grimaldis don't seem to have strong genealogical ties to other European dynasties.
That being said, the Prince of Monaco is a direct descendant of Stéphanie de Beauharnais; ditto for the King of the Belgians and the Grand Duke of Luxembourg. Since Eugène de Beauharnais (her second cousin) is a direct ancestor of the three Scandinavian monarchs, this would make the Beauharnais dynasty well- represented.
Did I miss any?
What about the Romanovs? Any czar(ina) with multiple sovereign descendants today? I know that the King of Spain would count, and that one day the United Kingdom will as well (since the Prince Consort is a direct descendant of Nicholas I).
As for Dom Miguel I of Portugal, I believe he has only three sovereign descendants today (i.e. Belgium, Luxembourg, and Liechtenstein). Any others?