Games like Second Source
Games like Second Source include Door Kickers, Men of War: Assault Squad 2, Cyber Manhunt, Papers, Please, and Beholder Conductor. We matched them by the tags players give Second Source on Steam (Strategy, Simulation, RTS, Puzzle, and Political Sim): closest matches first, and among equally close ones the better-known games.

Door Kickers
Shared: Strategy · Simulation · Top-Down

Men of War: Assault Squad 2
Shared: Strategy · RTS · Simulation

Cyber Manhunt
Shared: Simulation · Puzzle · Detective

Papers, Please
Shared: Simulation · Puzzle · Indie

Beholder Conductor
Shared: Political Sim · Simulation · Strategy

Mini Metro
Shared: Strategy · Simulation · Puzzle

CaseCracker
Shared: Puzzle · Text-Based · Simulation

Victoria 3
Shared: Strategy · Simulation · Political Sim

Beholder
Shared: Strategy · Political Sim · Simulation

Crusader Kings II
Shared: Strategy · Simulation · Political Sim

Wargame: Red Dragon
Shared: Strategy · RTS · Simulation

Mini Motorways
Shared: Strategy · Simulation · Puzzle

Democracy 4
Shared: Strategy · Simulation · Political Sim

Cyber Manhunt 2: New World - The Hacking Simulator
Shared: Puzzle · Detective · Simulation

Beholder 2
Shared: Strategy · Simulation · Political Sim

Dummynation
Shared: Simulation · Strategy · RTS

Call to Arms - Gates of Hell: Ostfront
Shared: Strategy · RTS · Real Time Tactics

The Escapists
Shared: Strategy · Simulation · 2D

RIOT: Civil Unrest
Shared: Strategy · Simulation · Political Sim

WARNO
Shared: Strategy · RTS · Real Time Tactics

Hearts of Iron IV
Shared: Strategy · Simulation · RTS

911 Operator
Shared: Strategy · Simulation · Real Time Tactics

Orwell: Keeping an Eye On You
Shared: Simulation · Political Sim · Detective

Unheard - Voices of Crime
Shared: Puzzle · Detective · Simulation