What do I do with Eridium after maxing out my SDUs?

So, I've bought everything available at the Black Market in Sanctuary but enemies still keep dropping Eridium. It's gotten to the point where my HUD no longer displays my total Eridium count whenever I pick some up.

Is there something else I can spend my Eridium on?

You can use Eridium to fight several of the late-game raid bosses. From the Borderlands wiki article on Eridium, the raid bosses requiring Eridium are:

• Terramorphous the Invincible (8 Eridium)
• Hyperius the Invincible (8 Eridium)
• Master Gee the Invincible (8 Eridium)
• Pete the Invincible (8 Eridium)
• Voracidous the Invincible (8 Eridium)
• Dexiduous the Invincible (90-99 Eridium)

Note that some of the bosses can be fought multiple times without having to re-pay the Eridium fee.

Tiny Tina's Assault on Dragon Keep DLC added a another good use for Eridium.

In Flamerock Refuge, visit the "Moxxi Grog & Girls" tavern. Inside you'll find two "Wut Wuuut" slot machines that run on Eridium.

To add to the answers prior to this one:

Tiny Tina's Assault on Dragon Keep DLC

• At the cost of 2 Eridium, you can activate shrines that give you various temporary bonuses: ammo, defense, melee, damage and speed bonuses.

  

• Using two dice instead of one when opening Dice Chests for better chances of getting a higher rarity item. Costs 5 Eridium.

  

• Accessing The Winged Storm for fighting Ancient Dragons of Destruction (raid bosses) in the mission, Raiders of the Last Boss, costing 20 Eridium for each access.

• You can give 5 Eridium to the queen* after you've rescued her (finished the final story mission).

  • She can be found in Flamerock Refuge. She has a chance of giving you a weapon whose rarity is either: green, blue, purple, a Hyperion E-tech Dart or Spiker, or a legendary weapon. There is also a small chance that she will produce a health, ammo, or money pickup instead of a weapon. Spoilers:

    * By queen, I mean Butt Stallion. And by "giving Eridium to the queen" I mean feeding Butt Stallion Eridium, and by the queen "giving you weapons/health/ammo", I mean Butt Stallion pooping or vomiting those out. More info here.

Ultimate Vault Hunter Upgrade Pack Two: Digistruct Peak Challenge DLC

You can use them to activate 'digistruct' loot chests for 5 Eridium.

Sir Hammerlock vs. the Son of Crawmerax DLC

• Once the main mission of the DLC is completed, as well the following side mission, Vault Hunters may return to The Deep, pay 20 Eridium, and fight The Invincible Son of Crawmerax the Invincible (raid boss).

• Costing 10 Eridium each, treasure syringes can be obtained from a dispenser on the docks near the Retirement Cottages. There are three green Varkid pods throughout the map. If injected with a treasure syringe, they will spawn friendly adult Varkids, which, if followed, will each unearth a treasure chest with higher than average chances to give legendary and pearlescent items. The pods regenerate in each new game session and can be re-activated for more loot. Syringes must be purchased each time and will not persist across sessions.

Commander Lilith & the Fight for Sanctuary DLC

• Butt Stallion will appear in The Backburner with Tiny Tina after completing the mission My Brittle Pony. Feeding her 5 Eridium will cause her to defecate or vomit a single piece of loot. She has a chance of giving you a weapon whose rarity is either: green, blue, purple, a Hyperion E-tech Dart or Spiker, or a legendary weapon. There is also a small chance that she will produce a health, ammo, or money pickup instead of a weapon. More info here.

• Fighting Haderax the Invincible (raid boss) in the mission, A Most Cacophonous Lure. Costs 20 Eridium.