A couple of the included accessory programs.

This is the ReactOS application manager, RAPPS. Unfortunately, a lot of these either don't work or had issues running on my system.

Probably because ReactOS has to put up with applications that embed IE for no reason, there is a copy of Wine Internet Explorer included.

ReactOS includes Firefox ESR 52 installable via the RAPPS program.

Of course you can install your own Windows applications...within reason.
This is the last release of the SeaMonkey web browser that runs under Windows XP. In theory, applications for later versions can work (there is a Windows 10 compatibilty mode, for example), but when I tried running the program it complained about things missing in various DLLs.
Of course the other problem is that ReactOS doesn't really have an add/remove programs option. Sure, there is one in the control panel, but it just takes you to RAPPS, and external programs don't show up there.