For your pleasure and my rapid deployment I present my personally endorsed and expertly curated short-list of sources for professionally appointed virtual appliances and environments:

• OSBoxes maintains an impressively varied repertoire of meat-and-potatoes server and desktop/end user installations of the most relevant flavours, lightweight performance-oriented distributions particularly suited to use in VM-optimized projects and embedded essentials like Android-x86, FreeDOS, Raspbian and more.
• Kali Linux stays on top of their VM images - get it from the source.
• Microsoft provides free images of Internet Explorer on Windows 7 through Edge on Windows 10. Web developers don't need to worry about appeasing Internet Exploder anymore but I don't need to tell you how valuable these images are to neglected and abused admins forced to wrangle legacy systems.
• Univention provides mostly free, mostly open source dockerized turn-key IT infrastructure and economically relevant virtual appliances based on its custom distribution Univention Corporate Server through an in-house app store model. You can choose to deploy a full-scale, centrally managed cloud solution composed of up to thousands of virtual machines or download a single virtual machine image directly from the web front-end to their marketplace, the Univention App Center and it will happily run, receive updates and benefit from the same standardized management capabilities as a massive cloud would all by itself. You can of course scale up later if you choose. It's.... actually pretty cool... (you guys hiring?)
• The best place to get Oracle Solaris images (among other Oracle offerings) is Oracle. Go figure.

Honourable mentions:

• Linux VM Images - we've already ticked almost all of these boxes, I'm including it since it's such a large collection but have never personally used it. Maybe today's the day you'll find something exotic in there.
• VirtualBoxes provides a few interesting non-linux images like Plan9 and MINIX.
• There is no better place to snag a weird Windows beta or nostalgia trip than Virtual Disk Images
• Microsoft provides evaluation versions of some editions of Windows and Windows Server; the CLI-only distribution of Hypervisor 2012 is provided here free of charge.
• Server 2008 R2 is available as a 180 day evaluation but Server 2003 R2's product page does not indicate a time limit (you tell me - I have better things to do! :D)
• Well, this doesn't look like a sanctioned release of Windows 98 SE but I won't tell if you won't.
• The Naval Postgraduate School's Center for Cybersecurity and Cyber Operations provides "Labtainers"- Fully packaged Linux-based computer science lab exercises with an initial emphasis on cybersecurity. God damn youth is wasted on the young. I wish i had time to play with this.
• Oh hey! TurnKey is still a thing. I'm afraid its integration/management features are about as compelling as its selection of software in comparison to UCS; particularly as the TurnKey Hub requires AWS... and I think at least half of the half dozen "fine publications" it touts having received coverage in went out of business while I was yet nae but knee-hight... >.>
• This disposable browser appliance had a lot more appeal 10 years ago when it was maintained around 300MB, the Firefox installation kept only a few versions behind and both virtualization technology and free, pre-configured virtual machine images were much harder to come by. I suppose I'm including it for nostalgia's sake, you're better off downloading the CentOS desktop version from OSBoxes.