Tapeworm was the great alternative/industrial supergroup that never was. It began in the mid-to-late 1990s as a Nine Inch Nails side project, a place for ideas that didn't quite fit the standard NIN MO, and an opportunity for live members Danny Lohner and Charlie Clouser to have more input. A slew of guests were involved or at least said to be involved. Clouser cited: Dave Gahan, Maynard Keenan James, Dr. Dre, Everlast, Phil Anselmo, David Bowie, Jaz Coleman, Page Hamilton, Zac De La Rocha, Richard Butler, and Marilyn Manson.

The project was quietly disbanded and has sustained almost mythical status ever since. But a small amount of material from whatever Tapeworm sessions occured has been released over the years.

I found it helpful to pull this material into a list in a way that's more organized than a bunch of Reddit threads, but allows for more speculation than a Wikipedia entry.

Confirmed Tapeworm material

Two full Tapeworm songs have been recorded and released, but not by Tapeworm:

• "Vacant," restyled and renamed "Passive," released by A Perfect Circle.
• "Potions," "covered" as "Potions (Deliverance Mix)" by Puscifer.

Confirmed derivative material

• The drum loop from "Starfuckers Inc." from The Fragile started as Tapeworm material, according to Charlie Clouser.
• Clouser also wrote that some Tapeworm material found its way into Helmet's Size Matters: "When I left I returned to Los Angeles and got back together with Page Hamilton, and we took a bunch of stuff that we had been writing together and turned it into the basis for Helmet's 'Size Matters' album."

Underworld soundtrack

This deserves its own entire section.

It's widely speculated that Danny Lohner used some Tapeworm material on the Underworld soundtrack. The NIN Wiki says that Lohner has confirmed this but does not provide a source and I've not been able to find any confirmation. The closest I've been able to find is the following from this interview, which I think is from before the soundtrack was released (because he says there are four Damning Well songs on the soundtrack, rather than only one):

Yeah, cause like behind the scenes people know that I work on other people's records and shit and um they'd heard a bunch of the stuff that I've done for various projects like a project called Tapeworm that we still haven't put out with Trent and Maynard and uh, just different things. And I've come to music that I've done that has not been released like me and Maynard have had a side project called Pucifer for a long time we've just not put anything out but we've got a bunch of songs. And uh, he came over to my house and he's like, 'Dude, you've got tons of cool shit. Do you want to be the music supervisor on this film?' And I said yeah, so I did it.

I could see how someone could interpret "I've come to music that I've done that has not been released [...] 'Dude, you've got tons of cool shit. Do you want to be the music supervisor on this film?' And I said yeah" as meaning that he used unreleased Tapeworm material on the soundtrack. But he also said in the same answer that "still haven't put out" Tapeworm, which suggests to me that he thought the Tapeworm material might still be released. At this point "Passive" hadn't yet been released on Emotive.

Asked about Renholdër, Lohner said "That's not even a project that's literally like I had some score pieces that I did for the film that they wanted to include on the soundtrack, and so they're just like little 30 second like things that go good with the picture for particular scenes. And we just threw them on there," which makes it sound like the Renholdër tracks from the soundtrack were made specifically for the film, not left over material from Tapeworm. Likewise, Lohner mentions that the "Disco King" remix with Maynard and John Frucante was created specifically for the soundtrack.

But the soundtrack includes the Page Hamilton/Charlie Clouser song "Throwing Punches," which was also included on the Size Matters album, which Clouser did say included Tapeworm material or at least Tapeworm-era material. So that lends credence to the idea that Underworld became a dumping ground for Tapeworm-related material and collaborations. But also keep in mind that Lohner explicitly says that Puscifer was a separate project (though it's entirely possible the project came about because of Tapeworm).

The soundtrack also features a song by a supergroup called "The Damning Well," featuring Lohner, Richard Patrick, Josh Freese, and Wes Borland. Richard Patrick and Josh Freese are both cited as potential Tapeworm contributors on the NIN wiki (but Patrick has denied involvement: here and here). Based on the above interview, it sounds like The Damning Well was assembled either specifically for the Underworld soundtrack or at around the same time, so while it might reasonable to think of it as a "spiritual successor" to Tapeworm, there's nothing to suggest that The Damning Well material had any relationship to the Tapeworm recording sessions.

Still, it seems like Underworld is the closest we'll ever get to a Tapeworm album.

Possible derivative/related material

• The intro drums from Saul Williams's "Convict Colony" might have been Tapeworm material, according a quote from Trent Reznor on the nin wiki. It's not clear to me what the source of that quote is, or if any other material from The Inevitable Rise And Liberation Of NiggyTardust! came from Tapeworm sessions.
• Everlast: "So Long." I'm surprised this track produced by Danny Lohner hasn't been mentioned before as a possible Tapeworm song. Everlast was mentioned as a Tapeworm contributor by Clouser. This track was released in 1999 on the End of Days soundtrack, so it fits the timeline for this to have been something that began as Tapeworm material then developed into a full song after the project disbanded. At the very least it seems like the collaboration between Lohner and Everlast could have come about because of Tapeworm. It sounds like it would fit on an album with "Potions" and "Rev. 22:20."
• There are two tracks that have circulated online that are referred to as Tapeworm songs. One, "It Fades," is reportedly a fan remix of the Quake song "Aftermath." I've not listened to either close enough to judge, but it does seem like it's not a Tapeworm song. The other, "Fading", does sound like it has Maynard Keenan James on it, but I'm not sure.

Other noteworthy material

• "Beautiful People" by Marilyn Manson. Yuck, right? But Tommy Victor of Prong was mentioned as a Tapeworm contributor, and has claimed that material he did for Tapeworm was used "on some Marilyn Manson stuff." Asked about this again in 2016, he said: "I’ve stolen or borrowed or been handed everything. Some people say I wrote the [Marilyn Manson] “Beautiful People” riff, but I don’t remember because I was so high on coke. Someone probably hummed it to me anyway."  I think it's a stretch, but that guitar part does sound like a Prong riff and he is listed in the "Thanks" section of the liner notes for Antichrist Superstar as "Tommy Prong." Zim Zum once said Victor only contributed about two seconds of noise to the album. My guess is that Antichrist was too early to have included anything that began as Tapeworm material, since Reznor was still talking about the project in 1997, a year after the album was released, but I can't discount the possibility entirely.
• Black Light Burns was a band that included Borland, Lohner, Freese, and Josh Eustis. It was essentially a successor to The Damning Well. Clouser contributed to the song "Lie" from the first Black Light Burns album Cruel Melody. By most counts this project was primarily driven by Borland so it seems very unlikely that any Tapeworm material ended up on this album, but there are moments of resonance with "Vacant" and "Potions" that make me think that parts of Cruel Melody might not have been out of place on a Tapeworm album.