Warsaw became the capital of Polish-Lithuanian Commonwealth in 1596. It was transferred there by king Zigmantas Vaza (Zygmunt Vasa) from Cracow. The place had been chosen as a mid-point between Cracow and Vilnius, respective capitals of Poland and Lithuania (in reality Warsaw is 450 km from Vilnius and 300 km from Cracow, this likely representing the larger Polish influence in the Polish-Lithuanian Commonwealth).
Polish-Lithuanian Commonwealth existed for two more centuries, allowing Warsaw to accumulate locations that remind of the Polish union with Lithuania.
Likely the most important among them is the Royal Palace (destroyed during WW2, rebuilt afterwards) which once housed the family of the monarch of "both nations". Polish-Lithuanian monarchs had little influence at the time and the real Power was vested in Seimas (Parliament), which also convened in the same palace (Great Hall). The world's second constitution (1791 05 03) was proclaimed there. Near the Great Hall ceiling there are coats of arms of Poland and Lithuania (Vytis) as well as the coat of arms of all the Voivodships (administrative units), of which three (Samogitia, Vilnius and Trakai) were within the area of modern-day Lithuania. They are represented by the Samogitian bear, Vilnius Voivodship symbols (which includes Vytis), and a plain Vytis representing Trakai (as the Voivodship had no coat of arms, using the Lithuanian one instead). The Palace museum has many historic maps of Lithuania.
In Warsaw St. John's Cathedral the final joint ruler of Poland and Lithuania King Stanislaw August Poniatowski (Lithuanian: Stanislovas Augustas Poniatovskis) is buried. There are also commemorative plaques for Vilnius University and Poles of Lithuania. Moreover, the Cathedral is also the final resting place of Gabriel Narutowicz (Lithuanian: Gabrielius Narutavičius) who was born in Telšiai to a family of somewhat Polonized Lithuanian nobility. They reflected the final division of a "Polish-Lithuanian nation" as Gabriel Narutowicz's own brother Stanislovas Narutavičius became one of twenty signatories of Lithuanian declaration of independence on 1918 02 16. The Polish-Lithuanian relations reached their nadir soon afterwards with the Polish occupation of Vilnius region when entire Eastern Lithuania became ruled from Warsaw once again (1920-1939). References to the era in Warsaw plaques may still evoke controversy among Lithuanians.
Warsaw street names remind of other Polish-Lithuanian era figures. The only difference from similar memorials you may find for them in Lithuania are in names: Poles use Polish versions while Lithuanians use Lithuanian ones. For example, Emilii Plater in Warsaw is the same famous female fighter against Russian domination that is known as Emilija Pliaterytė in Lithuania.