If you've ever had a weird startup idea, you're not alone. There's precedent when it comes to the world of strange startups--and some have proven massively successful.

Many of those on this list were founded by people who had a simple but compelling problem they needed solved. They couldn't find a good solution, so they generated their own--and then founded a company based on it.

Here are seven weird startup ideas that became successful:

1. I Do, Now I Don't

Josh Opperman's was engaged for three months when his finacée left him. He was devastated--a feeling compounded by his attempt to return the ring to the jeweler. They said they'd give him $3,500 for the ring he got for $10,000.

He established I Do, Now I Don't to rectify this, and it became a raging success. Like Craigslist for fine jewelery, it offers sellers a way to get fair price for their rings, and buyers to get a big discount off retail prices.

It also solves the problem of quality assurance: the site acts like escrow for the rings. Once an item sells, the company holds the money, has the jewelry checked out by a certified gemologist, and only passes the item on if it matches the quality outlined by the seller.

As for Opperman's personal life, he's now married with children. "I definitely wouldn't change a thing," he says. "I have looked back and I'm actually glad [the breakup] happened... Of course I wouldn't have the business if this hadn't happened to me." 

2. BrewDog

In 2017, the founders of unique beer company BrewDog released a second version of their ultra-alcoholic ale (it was 55 percent alcohol by volume, or 110 proof). It was called The End of History, and in 2017 it sold for $20,000 per bottle.

This is likely in part due to the unique packaging--the bottles are placed in taxidermied roadkill. Yes, you read that right. Some of the squirrels in question wear kilts.

According to the company, "[The End of History] is an audacious blend of eccentricity, artistry and rebellion; changing the general perception of beer one stuffed animal at a time."

3. Spud Pickles

Spud Pickles is a Utah-based app development company that creates and maintains custom apps for individuals and businesses.

Sounds normal so far. But one of Spud Pickles' best-known suite of mobile apps is called Ghost Radar.

They're apps for ghost hunters. 

Yes, these popular apps are used by both professional and amateur paranormal investigators alike. A disclaimer on the app description says, "since results from this application cannot be verified scientifically the app should be used for entertainment purposes." That said, app reviews abound with ghost sightings and other otherworldly activity.

4. Diedinhouse.com

Speaking of weird and creepy startups, this company was created because its founder "found out that someone died in my house before I bought it."

He had assumed this information would have been part of the disclosure process, but discovered that in most states your realtor or broker is not required to tell you whether a murder, suicide, or other violent death has taken place in your house.

Enter Diedinhouse.com. For ~$11.99, it will tell you whether someone died in your house--whether you're already live there or are considering purchasing it. 

5. CoolMiniOrNot

This startup began as a website where Dungeons & Dragons nerds could show off their elaborately-painted D&D figurines. It took off. 

The founders eventually monetized by designing and selling their own games on the site. They used Kickstarter as their funding platform, raising over $35M on the platform over the course of 27 campaigns.

6. The Pet Loo

When it's 10 degrees out and you live in an apartment, it's hard to get excited about taking your dog out for a walk. Or maybe you work long hours and it's stressful to make it home to let your pet out. Many people skip social plans because they know they have to get home to let the dog out.

The Pet Loo is a startup that solves this problem with a practical solution--it sells a square of fake grass that sits on a waste containment system. Urine soaks through the fake grass, which is made from an odor-reducing material. Many pet owners prefer it to pee pads.

7. Reef Balls

In the late 1980s, Don Brawley was a college student and an avid diver in the Florida Keys. A firsthand witness to the degradation of the coral reefs, he wanted to help. So he and a friend developed what would eventually be called "reef balls"--eco-friendly concrete-like molds that mimic nature's coral growth.

Since Eternal Reefs' first project in 1992, over 700,000 reef balls have been placed in the ocean in more than 70 countries worldwide. Reef balls have become the gold standard for coral restoration, habitat development projects, and fisheries programs all over the world. 

Equally as remarkable is the addition to the organization that took place when Brawley's father-in-law said he wanted to have his cremated remains put in a reef. "I can think of nothing better than having all that action going on around me all the time after I am gone," he said. "Just make sure that the location has lots of red snapper and grouper." 

Now Eternal Reefs offers reef ball urns to those who want their final resting place to double as a meaningful contribution to the marine environment. Sailors, divers, fishermen, environmentalists and more have chosen to take comfort in knowing that their connection to the sea will be eternal.