Greg Kelley's improvised trumpet noise of Trumpet (june 2000 - Meniscus, 2000) re-framed trumpet improvisation for the age of noise using the trumpet as a source of grating sounds. The natural consequence of that approach was Field Recordings (july 1998 - INtransitive, 2000), a collaboration with Tatsuya Nakatani and Curt Newton, which experimented a fusion of concrete music and free-jazz. Forlorn Green (march 2001 - Erstwhile, 2002) was his first collaboration with digital/electronic musician Jason Lescalleet. Empty Gesture (Freedom From, 2001) contains 19 brief pieces for field recordings, electronics and trumpet.

Kelley also recorded music for trumpet and electronic sounds: the 37-minute piece of If I Never Meet you in this Life Let me Feel the Lack (october 2001 - Rossbin, 2002) and the 36-minute piece of I Don't Want to Live Forever (Little Enjoyer, 2005).

Greg Kelley joined saxophonist with Bhob Rainey, a fellow Bostonian, to launch the project Nmperign, which debuted with the five lengthy improvisations of Nmperign (Twisted Village, 1998), aka 44'38"/5, followed by This Is Nmperign's Second CD (Twisted Village, 1999), mainly devoted to the 16-minute noise abstration Not Yet Winter Autumn Still. In Which The Silent Partner-Director (Intransitive, 2000) contains a collaboration with electronic musician Jason Lescalleet, namely the 14-minute Such As A Refrigerator Or Even Happiness, an electro-acoustic nightmare in the old vein of Karlheinz Stockhausen, as well as Jason Lescalleet's own 14-minute computer piece Or How Misunderstandings Are Born.

That program of absurd, cacophonous, irrational duets was continued on Nmperign (Selektion, 2001), More Gloom More Light (august 2002) with Gunter Muller, Salt Peanuts (American Tapes, 2003), We Devote Every Effort To Offer You The Best That You Deserve To Have For Your Enjoyment (Siwa, 2004), containing two lengthy pieces of soft cryptic noise, Thanks Cash (Sedimental, 2004) with Axel Doerner and Andrea Neumann, the double-disc Love Me Two Times (Intransitive Recordings, 2006), recorded between 1999 and 2004, documenting more collaborations with Jason Lescalleet ranging from 40 seconds to the 23-minute White Wall, and Ommatidia (Intransitive, 2009). The one-sided LP Selected Occasions Of Handsome Deceit (2009) collects the very first recording by Nmperign, on which the duo played with drummer Jake Meginski.

Rainey also released the solo album The Withered Grasses (1999) and I Don't Think I Can See You Tonight (2007), a collaboration with Ralf Wehowsky.

BSC was formed by Bhob Rainey with Greg Kelley on trumpet, Mike Bullock on double bass, James Coleman on theremin, Chris Cooper on prepared guitar, Vic Rawlings on cello & electronics, Howard Stelzer on tapes, and Liz Tonne on vocals, and documented on Good (Grob, 2003) and 23% Bicycle and/or Ribbons of the Natural Order (2010).

Graveside Doles (Ultra Eczema, 2006) documents a (heavily cacophonous) collaboration between Kelley and Alex Nielson.

Sannyasi (may 2001 - Wet Paint 3001, 2002) was a trio with saxophonist Paul Flaherty and drummer Chris Corsano.

Cold Bleak Heat was a supergroup of the Boston avantgarde formed by saxophonist Paul Flaherty, Nmperign's trumpeter Greg Kelley, No Neck Blues Band's bassist Matt Heyner, Six Organs Of Admittance's drummer Chris Corsano. They released the exuberant improvisations of It's Magnificent, But It Isn't War (august 2003) and especially Simitu (august 2003), with the 21-minute Mugged by a Glacier, that straddle the border among soundsculpting, minimalism, free jazz, punk-rock and funk (both were recorded at the same session in 2003).

Kelley was also playing with the psychedelic band Heathen Shame, with the chamber music Undr Quartet, and with the rock band Life Partners. Kelley also recorded three EPs titled The Peeled Ones I-III (2005), two for synth and one for guitar. At the same time Kelley recorded the 16-minute piece of the EP Is Uncomfortable (American Tapes, 2005). Kelley's The Traditions of the Past Cannot Be Retrieved (8mm, 2006) colleects recordings of 1997-2005, notably the 31-minute Piezos (1997). Kelley also played in a Scelsi tribute band, documented on One Hour As Something That Didn't Turn Out The Way I Intended It To (Chocolate Monk, 2007). Kelley recorded the solo trumpet album Self-Hate Index (Semata, 2008) and Religious Electronics (2008) collects two lengthy solo improvisations of the same period: Despair is Sin and O Lord The Star Torments Me.

In 2009 Rainey relocated from Boston to New Orleans.

Resolution (august 2010) documents a collaboration between Greg Kelley and Olivia Block.

From Null Lands Led, Starrily (2017) features Bhob Rainey on synthesizer, Matt Mitchell on piano, Chris Cooper on sounds, Liz Tonne on vocals, and James Ilgenfritz on bass.

Kelley released the 17-minute trumpet solo Threnody to the Limits of Musical Expression (2023).

Distractions For Trumpet & Ajaeng (july 2025) documents duets with Yoona Kim (Korean zither ajaeng).