There must 101 ways to accomplish this task, ranging from uber simple to very complex, from ridiculously insecure, to something even the NAS might find challenging. And since we don't know your skill level, or exactly who you'll being sharing this with (family, friends, colleagues) and the varying levels of trust that implies, and whose own skill levels and access tools we also don't know (PCs? tablets? smart phones?), well…, let’s just say, it gets complicated. And as a rule of thumb, the easier it is to implement, the more it costs. So if you’re looking for the freebie path, it can prove daunting, at least if you want to do everything possible to protect your security and privacy.
So erring on the conservative side and assuming the absolute worst, my recommendation is that you NEVER expose your system folders/files directly to anyone else. Instead, move/copy the files you wish to share off your PC to a device intended ONLY for sharing. And only use a tool that provides web based access, and over secure protocols anyone is sure to have, w/ a good mix of platform support (PC, Mac, Linux, iOS, Android). To that end, I’d suggest something like a PogoPlug w/ multi-user sharing. There may be other similar devices, but that one comes immediately to mind.
With the PogoPlug, you can attached one more USB external storage drives (flash or HDD) and copy files in and out as needed. And users can access them via the web. And if you’d prefer, you can even upload them to the cloud to minimize the impact to your own upload bandwidth (so multiple ppl aren’t uploading the same file). You get 5GB for free, but can pay for more if you like.
Understand, I’m not suggesting you couldn’t find some other methods involving more complex setups, even at little to no cost, but again I’m looking at this VERY conservatively and thinking what’s the easiest and most foolproof solution. At least until you convince me you’re prepared for more complex solutions.