Well, it turned into one of those threads… 
I was hesitant about what to really make of the premise “What other apps do you pair with DEVONthink” – not because it’s not an interesting question. But to me personally, it implied the deeper question of ‘where does DT really sit in your/one’s overall workflow’. There is a lot of semantics just in ‘pairing’, I guess.
It’s totally fair, but I think like some discussions, the most active bunch of people discussing this are people that work on the premise that DT is ‘the one’ base app for their ‘knowledge work’ (and often with more than a foot in coding and text-centric work). And there is plenty of reason for that. But I also think for another larger part of the actual user base, DT is ‘one among others’ (assumption!) – … for different reasons.
But to turn this general thought/impression to something intersecting with the substance of the discussion – but also to point out questions (or: potentials) rather implicit in such discussions, here are two sharings from my use crossing into all this:
For one, I am now pretty much at using DT and Capacities (was mentioned as Task manager ) side-by-side. This brings up other topics relevant here: what is the relation of ‘the other apps’ used – auxiliary tool, linking tool (for other/outside workflows), parallel solution, simply premised because of other use domains etc. etc.? Then, as I am currently rethinking the homeostasis between Capacities and DT (given its MD-editor improvements and clever AI-onboarding), I think there is the whole question of where people use apps because of what they feel are shortcomings/misses in DT, while they still use it as central hub.
The reasons for using Capacities side-by-side are mainly two: better integrated UI (with things like neat dashboards) and editing experience (to some degree); an overall feeling to have all modes and ‘documents’ integrated into a coherent ‘hyperdoc’ feel (where you can easily interlace different elements via embedding). In a way, it’s very Engelbart, just for the personal sphere and with coherent UX ). – See also these 2 recent threads: [1 & 2)
– I am not sure yet where this will end. But given Capacities now goes for automatic downloads and DT’s power to index and ‘interlace’ I can see a future where these 2 will continue to co-exist kind of ‘on par’.
Then, secondly, I felt while some image (‘visual’?) apps appeared, this aspect of multimodality triggers a whole other route of thinking, as to the question of “pairing”.
As recently again stated, I am using images/photos not ‘only’ as illustrations or ‘pure images’ (I do that too, of course; so no disrespecting), but I actually use photos (indeed media) as rich and layered documents – which is pretty much the explicit core use case of DT.
For that, I need other ‘companion apps’ to do things that DT can’t do (it’s a mix of: ‘DT can’t do anything’ and ‘DT has a text- and code-bias’ here for me). So here I am using apps like Bridge, or the aforementioned Graphic Converter to really enable DT to coherently work with ‘photos as documents’ the way it does.
Here again, I feel another question lingers: maybe there are some apps and interoperabilities that are missing in/for DT, as this whole ‘pairing’ is really rather a ‘stitching’ (see this thread)?
After these more basic thoughts on the whole framing of the thread, and its course of discussion, here are some of the apps that I can/would add from my side to the impressive list of @Korm:
– Safari (after a liaison with Orion); Friendly + Multitab for Browsing the ‘social web’
– Capacities
– TheBrain (basically great because it provides an interactive + very unique mapping mode, and can also attach/index documents, plus works very well with deeplinks, that is DT references…)
– QSpacePro (better Finder; also lot of things integrated, like Dropover kind of functions etc.)
– Raycast (everything app)
– Keyboard Maestro (like every one 
)
– Elephas (local, OS wide AI + writing assistant)
– Preview (a lot!); Movist Pro being its analog for videos…
– Typora (real pleasant MD editing); Ulysses (for some intermediary ´handling’ tasks rather than real writing, notetaking)
– PDFExpert
– Things (not as task manager, but as quick, structured note + reference pad)
– GoodTask (tasks, calendar integration, daily desk + operational notes)
– XMind (for me more versatile/graphical compared to MindNode; SimpleMind)
– ExcalidrawZ (MacOS specific solution to use Excalidraw; scheming, concept boards etc.)
– IA Presenter (Journaling; easily turn MD notes + media into graphical, semi-layouted documents + presentations)
– Eagle (quick access graphic repository)
– Lex.page/Scrivener for writing; still flirting with Speare…
– HoudahSpot, Foxtrot and PDFSearch (for searches outside of DT; PDFSearch uniquely allowing to search for relevancy-per-page!)
– Perplexity, Cove, Kagi (Web research)
– InoReader/Unread (all things RSS + feed related; reader – again parallel to DT); iCatcher for Podcasts
– Affinity Suite, Capture One, On1 (graphics + images); also NIK Collection + CameraBagPro… – as mentioned before: Bridge, Graphic Converter and to some extent hotoMechanic (plus EXIF Editor) for treating photos as documents (metadata, contact sheets etc)
– FinalCut + Kyno (video – editing + video as content documents!)
– Dropbox + Dropmark (sharing, bookmarking, video hosting etc.)
– Bear (nostalgia for universal aesthetic notebox)
– …
There are a lot more if we go into utilities. So, just some maybe:
– Text Workflow (standard + custom text transformations)
– Scaffold (reorganizing, templating, renaming folder structure via text + lists)
– Text Sniper (getting text from simply anywhere on screen)
– SyncFoldersPro
– MakePDF (quick document fusion)
– TypeFace (mentioned before)
– Dropover, Tot, Antinote (thx, @jonmoore !)
– Kompressor, PhotoMill
– Lut Manager
– Aiko (local transcription)
[added]
– Start [app + project spaces launcher]
– Whisper [transcription w/ watched folders]
– Sidedesk [visual custom desktop; not perfect but one of the only filling this huge gaping hole of need]
– …
oh, and Simple Weather, CoCoCal and OSMAnd Maps… just to stop somewhere.