When you deployed, what were some resources, organizations, or services that you wish you had known about?

Without hesitation, it would be Military OneSource*.

For those not familiar with it, Military OneSource is a DoD-funded resource that I can best describe as "1-800-Help me with anything!" for military and their family. It was created 20 years ago by DoD to answer the need for a centralized assistance program to service members and their family.

For example, they have extensive* pre-deployment/deployment/reunion & reintegration articles, checklists, resources, etc. for the servicemember and their family.

Truth be told, I'm a bit of an evangelist for them as I found them instrumental and have referred dozens of Soldiers over the years to their resources.
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* Military OneSource - https://www.militaryonesource.mil/
* Pre-, during, and post-deployment page - https://planmydeployment.militaryonesource.mil/

Back in the day, I wish I knew more about the VA, the USERRA, and the Veterans Center (for mental health and other programs they provide).

Disabled American Veterans (DAV). You don't need a disability to join and they do a lot more than just advocate for disabled veterans. And if you ever feel like you need to file a claim they will help you do it so it's not so daunting. They really try to improve the whole experience of being a veteran - everything from disaster relief to transition services. For example, they helped sponsor a trip last year to send myself and seven other veterans to hike Mt. Kilimanjaro just because of the mental and physical benefits as well as veteran camaraderie it fostered.