https://github.com/quartzjer/js0n
Ugliest interface possible, but does what you ask. Zero allocations.
http://zserge.com/jsmn.html Another zero-allocation approach.
The solutions posted above all do dynamic memory allocation, hence will be inevitably end up slower at some point, depending on the data structure - and will be dangerous to include in a heap constrained environment like an embedded system.
Benchmarks of vjson, rapidjson and sajson here : http://chadaustin.me/2013/01/json-parser-benchmarking/ if you are interested in that sort of thing.
And to answer your "writer" part of the question i doubt that you could beat an efficient
printf("{%s:%s}",name,value)
implementation with any library - assuming your printf/sprintf implementation itself is lightweight of course.
EDIT: actually let me take that back, RapidJson allows on-stack allocation only through its MemoryPoolAllocator and actually makes this a default for its GenericReader. I havent done the comparison but i would expect it to be more robust than anything else listed here. It also doesnt have any dependencies, and it doesnt throw exceptions which probably makes it ultimately suitable for embedded. Fully header based lib so, easy to include anywhere.