Disclaimer : This is definitely a personal list. I'm happy to hear about others, however.
Synths/Samplers/Instruments
Yes, that is really its name. And it is a good name. If all you want to do play around with the sample libraries available for it, then it is great. If you want ot make your own sample packs, be ready to get your hands dirty. A great source for libraries is the PianoBook initiative (https://pianobook.co.uk). The search interface lets you filter so that you only see the libraries built for Decent Sampler.
A truly capable synth with a powerful modulation system and wave shaping tools. Along with the free version are several paid tiers that get you more presets. The engine and capabilities are the same.
Another almost overwhelmingly capable synth with a clean sound. And it supports the new CLAP plugin standard as well as VST3, AU, and LV2 for the Linux users.
A growing set of free libraries for their SINE player. Orchestral Tools is a respected VST instrument maker in the orchestral/cinematic space.
A free, open-source modular rack synth with over a 1000 modules to play with. Yes, it is a plugin.
A full orchestra with multiple articulations per instrument. Great way to get the "feel" of a large library. Paid upgrades are available, of course, with even more instruments and articulations.
A synth "sampler" - presets that span a large range of Arturia classic synth recreations. Includes selections based on the Wurlizer, DX7 FM synth, and much more.
Effects
I admit. I'm a bit of a fanboy for the Kilohearts ecosystem. But the free "essentials" is a great starter kit of effects plugins. It includes a very capable delay, chrous, compressor, and much more. To be sure, your DAW probably has many of these as stock plugins. But there are also some more unusual effects like tape stop, trance gate and reverser. While it is not free, I would suggest you pick up Snap Heap which allows you to use the effects as a part of a multiFX system complete with an incredible modulation system.
A great assortment of very good plugins. Two of the more useful are the Loudness Analyzer and MAnalyzer - a spectral analyzer. But look at the rest. You'll probably find something interesting.
Really, really great reverbs with a bunch of personality. Their free Super Massive is a must have. And if you have the change to cough up for some of the paid ones, do that too.
They have two free plugins that are really good for making glitchy sounds for a number of different genres. Go to the bottom of the page to see both.
Want the subtle warmth of tape saturation? Or want to add tape wobble or noise like a worn out tape? Cymatic has you covered with this free easy to use plug in.
A four-band parametric EQ that has dynamic processing. So each band can also act as a compressor. It can be a sibilance tamer or mud remover.
Turn mono signals to pseudo-stereo. It claims to maintain mono compatibility. Has a built-in tunable high-pass filter to allow you to exclude low frequencies from the effect.
Includes a very nice transient shaper and other solid if limited plug-ins.
Utility/Analysis
Great on the master track to help tune in loudness for streaming or whatever. And with true peak monitoring, you can make sure there is no unintended clipping.
Sounds/Libraries
An extensive collections of free sound libraries for Decent Sampler, Kontakt, and other samplers. While the quality is variable, there is certain be something quirky that will inspire you. Also, each library also has all the samples unlocked, so you can use the raw samples for stretching, chopping, whatever.
Lots of free samples such a drum hits, foleys, vintage synth and more.
Huge library of sound effects clips and a UI to blend them and warp them.