I'll give names and brief descriptions
Ferrothorn - Weakens the impact of fire, walls opposing rain abusers to oblivion, and is a solid pivot in or against any weather but sun. Provides an Electric buffer.
Mega Manectric - Fast, can destroy opposing Ferrothorn, one shots Charizard Y, and is pretty immune to Thundurus.
Thundurus (Incarnate and Therian) - Strong choices, Incarnate being one of the driving forces of OU and can T-Wave a Swift Swimming opponent getting out of hand. It also takes care of Tyranitar via Superpower and Charizard Y via Thunderbolt. Thundurus-Therian can take out opposing Thundurus-I, which are big threats to rain.
Zapdos - Adds Defog utility while being able to wreck Ferrothorn and Charizard Y. Yes, Heat Wave can KO almost all Ferrothorn outside of rain. Also checks Pinsir, etc.
(Mega) Scizor - Either Mega or non-Mega Scizor can enjoy the buffer vs fire, as well as provide good support against Sand, or in Mega Scizor's case, can straight up sweep. Also add Defog support and can surprise standard Heatran if you EV correctly and OHKO with Superpower.
Mega Ampharos - 4x resists electric, resists water, resists grass, all of which threaten rain. RestTalk Volt Switch and Dragon Pulse allow it to absorb status as well as it can forgo RestTalk for a offensive and bulky Heal Bell.
Tornadus (Incarnate or Therian) STAB Hurricane + Heat Wave + U-Turn + Superpower/Focus Blast and Regenerator or Defiant to back it up? Good choice. Heat Wave once again allows us to take on the pesky Ferrothorn outside of rain, as well as Superpower dealing damage to it in the rain. Bulky AV Tornadus-Therian or LO offense, or even Tornadus-I for a pseudo Thundurus.
Heliolisk - Dry Skin works out well as well as STAB Electric + Surf. Frail though. Life Orb is a good bet due to Dry Skin.
Toxicroak - Can use Dry Skin to run a good Sub set, maybe even SubPunch if it suits you. The water immunity and diverse typing allows you to use a fighting type and gives you a Fairy type check.
Bronzong - has only Ghost and Dark weaknesses in rain if you used Levitate, and can provide an auxiliary rain setter if needed.
Metagross - Packs a large punch and enjoys the rain's dampening of fire moves when it has the movepool to take out common fire types, just not the speed.
Sunkern - Kills all
Numel - Invalidates any right for the opponent to win
Magikarp - Swift Swim, Magikarp's Revenge op imo.
Of course, you can find more by looking through the list. This is a pretty solid OU coverage list, though.