I love Ben Aaronovitch's Peter Grant Series and Glen Cook's Garrett novels, and well, I like The Dresden Files. So what other magical private investigators are out there? Please no supernatural romances.
Dead Witch Walking and subsequent books (The Hollows series) by Kim Harrison? I think they do a better job, in some ways, of doing some of the things Butcher is doing with Dresden, but then Harrison doesn't seem to me to be trying to be noir at all.
Caveat: I've really only read the first one.
It's an alternate modern world setting, mildly post-apocalyptic, sort of Shadowrunny but not at all punk that I recognized. I didn't feel like it was a romance, although there were romantic elements in it to, it seemed to me, the same extent that such elements exist in Garrett and Dresden books.
The Hollows series by Kim Harrison is very definitely about an investigator, who works privately in the aftermath of an apocalypse in the 60s, caused by early genetic engineering experiments. It's sort of interesting, but there are a lot of potentially-triggery scenes later on.
The early Anita Blake novels are probably what you're looking for, but after about book 6, it gets very sex-oriented.
Felix Castor books by Mike Carey.
THey;re as close to Dresden as you can get without being Dresden, throught more low-key and darker.
Fix isn't a wizard, he's exorcist. Whiich means a person with a natural talent at summoning and banishing ghosts (and demons).
He lives in fifteen minutes in to the future where ghost became more common and their existance has been versified by science (throught there still a lot of people who don;t believe in them).
Fix a guy who day job was get rid of ghost, throught he's having a crisis of profesinal faith, as he's starting to wonder where exactly he;s sending the ghost to.
And ghost come in interesting varieties:
You usual plotergiest, Loup-Garou (not necessarily Werewolf, but ghost inhabiting animal bodies and forcing then in human shapes), Zombies (not the brain eating kind, juts ghoist who possesed their dead bodies, and now are living in dead bodies with their former intelligence, if they didn't go crazy) and aforementioned demons.
Fix gets involoved in various supernatural mysteries, and through not a actual PI, he ends up doing a lot if investigating.
Benedict Jacka's Alex Verus series owe a debt to Butcher, and acknowledge it in the text, but are darker and less gonzo than later Dresden books get. Plus the magical society, with hostile but not actively warring Light and Dark governing bodies and a protagonist who wants nothing to do with either, allows for a lot more noir than the White Council in Dresden. Recommended.
How close to the Hammett/Chandler archetype do you want to hew? Barry Hughart's Master Li (Bridge of Birds, etc.) is a private eye by trade, but the setting is faux-Tang China the stories are anything but hardboiled.
For something completely different, I recommend the Lord D'Arcy books and stories by Randall Garrett. The books are set in an alternate history where magic is the science of the day and mundane science is considered a dubious and arcane pursuit. Lord D'Arcy is a special investigator for the Duke of Normandy, and is clearly a Sherlock Holmes pastiche. His Watson is a forensic sorcerer named Sean O'Lochlainn. Magic features in every mystery, though Darcy himself doesn't use it. Garrett wanted to write mystery stories featuring magic which didn't "cheat" the reader by declaring "magic did it" with a handwave. He wanted instead to create magic which worked according to rules, allowing deductions to be made based on those rules.
He did a pretty good job, IMHO. They're fun reads.