I realize we just announced Jagged Alliance: Crossfire earlier today, but if you'd rather not wait for some more turn-based RPG/strategy goodness, you can immediately dive into S2: Silent Storm Gold Edition on GOG.com for $9.99. The description doesn't specifically mention that the Sentinels expansion pack is included, but the "two complete games" line suggests that it is:
The Silent Storm series brings you two complete games that both provide with you a wealth of turn-based combat against the backdrop of World War II and beyond. Itâ€™s the year 1943 and you must lead a group of operatives as they assault against the enemy, wreak havoc on their operations, and find out what new power threatens both sides. After the dust settles from the warâ€™s end, you must find and destroy the remnants of the organization known as â€œThorâ€™s Hammer.â€
Silent Storm Gold features a fully destructible 3D world with ragdoll physics and non-linear gameplay. Play the game your way: engage in stealth and quietly take out your enemies or take the more direct route with a rocket launcher. Choose from over 40 operatives in each game, for a total of 80 unique personalities and skills, and arm them with over 75 different authentic and painstakingly-researched weapons.