KTOG is a good place to start: http://www.ktog.org/cgi-bin/yabb2/YaBB.pl?board=499
On my RFB #T0029, I've added a few items:
-6x ACOG from Bear Basin
-LaserMax UniMax green LASER from Natchez
-LAPUA subsonic, Winchester Q3130, etc. ammo from MIDWAY USA
-Cheekrest (stick-on) from Caldwell Shooting Supplies
-Bayonet from Kel-Tec CNC
-Bayonet lug adapter (in R&D with a prototype)
-KYDEX adjustable cheekrest (in R&D)
-Discrete Case (COFFIN Case for electric guitar) from Musician's Friend
-QD pushbutton sling mounts from Daniel Defense
-Singlepoint sling from Blue Force Gear
-Custom flat ground wrenches (for FH/jamnut removal) from Hooper LE Supply
-SILENCER (CYCLONE) from AAC (pending)
-FN FAL Herstal 20rd metric magazines (10 packs, nos, niw) from STURMGEWEHR etc.
-Pickup some pure white Lithium grease, Permatex Anti-Seize, EEZOX etc. to keep things running smoothly.
Some of the other owners of RFBs have installed different muzzle brakes, FHs, GripPods, bipods, accessory rails, BORS, a wide variety of scopes & reflex dot sights, BUIS, special coatings/finishes, flashlights...
The RFB is a pretty special weapon & extremely well designed for ambidextrous shooting. That's a lot of .308 punch in a very compact package. Kel-Tec has a virtual monopoly on this niche market & has thoroughly debugged the weapon prior to release.