Knightly orders. Orders are practically a necessity if there is a cavalier in the party. Unearthed Arcana states, "In order to become a cavalier, a character must be in service to some deity, noble, order, or special cause," (pg. 14). Even without cavaliers in the setting, orders are a nice support group for a fighter to join; a secular order is the equivalent in many ways to a guild and is more accurate than the generic "adventurers guild" of many games. My focus is on religious orders, primarily because I am a religious person in my personal life. Your mileage may vary.
Knightly orders provide structure and are not necessarily religious in nature. Looking at Greyhawk for inspiration, there are religious orders that are similar to Earth's Templars such as the Brotherhood of the Lance Unbroken (Heiroenous), the Order of the Six Severed Hands (Hands), and, as a historian one of my personal favorites, the Knights of the Book (Delleb). However, other orders are secular in nature, such as the Knights of the Hart.