Walther PPK
7.65x17mm Browning SR (.32 ACP) or 9x17mm Short (.380 ACP) or .22 Long Rifle or 6.35x15mm Browning SR (.25 ACP)
Released in 1931, it was popular with European police and civilians for being reliable and concealable. In WW II it was used extensively by the German forces (Adolf Hitler killed himself with his own one) and was made famous as James Bonds weapon of choice.
It was one of the first reliable double-action semi-auto pistols and is still available today in .380 or .32 ACP