Common words:
* Please
* Plenty
* Pleasure
* Pledge
* Pleiades (a star cluster)
* Pleurisy (a lung inflammation)
* Plexiglas (a type of plastic)
* Pliers (a tool)
Less common words:
* Plebeian (commoner)
* Pleiad (a member of the Pleiades)
* Plethoric (excessively full)
* Pliable (easily bent)
* Plight (a dangerous, difficult, or unfortunate situation)
* Plump (fat or round)
* Plume (a feather)
* Plunk (to drop something with a dull sound)
* Ply (to work at something steadily)
* Plebe (a freshman at a military academy)
Let me know if you'd like more examples, or if you're looking for words with a specific meaning!