Here's some stuff I can think right off the top of my head.
GLFW is similar to SDL2, but much simpler functionality (no renderer):
https://github.com/glfw/glfw
SDL2 and GLFW are two of my favorite libraries for figuring out platform specific bits for windows/events/input.
miniz - single file zip manipulation library:
https://github.com/richgel999/miniz
c99-snprintf - single file portable implementation of snprintf function:
https://github.com/weiss/c99-snprintf
c4 - pretty cool subset of C interpreter with just 4 functions (it can interpret itself)
https://github.com/rswier/c4
webby - two file (h/c) tiny http webserver for game development:
https://github.com/deplinenoise/webby
mongoose - two file (h/c) feature rich webserver (http, websockets, json-rpc, and more), useful for embedded environments, unfortunetaly it changed license from MIT to GPL at some point:
https://github.com/cesanta/mongoose
xbyak - cool C++ header library to JIT assembly code:
https://github.com/herumi/xbyak
asmjit - similar to xbyak, but bigger library:
https://github.com/kobalicek/asmjit
nedtries - C and C++ header file that implements efficient in-place bitwise binary Fredkin trie algorithm (std::map/set alternative):
https://github.com/ned14/nedtries
jsmn - two file (h/c) super minimalistic C json parser (actually it's more tokenizer than parser), doesn't use any dynamic memory allocations:
https://github.com/zserge/jsmn
sxml - same as jsmn, but for xml files:
https://github.com/capmar/sxml