Flashlight expands the capabilities of the Spotlight app, which allows you to search your Mac for specific files or file types within Apple's operating system.
Open source: There are many free programs on the marketplace, but the vast majority of them are only free in certain ways. They're constantly trying to upsell you on premium versions or want to nickel and dime you to unlock certain parts of the feature set. Flashlight for Mac is open-source software, meaning it is truly and genuinely free to use.
Expansion of extant tools: The nice thing about Flashlight is that it doesn't reinvent the wheel. Instead of requiring that you install a completely new program in your system, it expands on the capabilities of a tool that you already have and probably use. It turns Spotlight into a much more powerful search tool, allowing you to search for weather information, send messages, and search specific online services such as Reddit or Wolfram Alpha.
Too wide open: Flashlight's capabilities are huge, but you have to be the kind of person who likes to tinker with a program in order to get the most out of it. If you just want an easy to operate, out of the box widget, this isn't going to appeal to you.
Still too early: It feels in every way like an early version, and using it for any period of time will reveal that it still has some kinks to work out.
If you're interested in transforming Spotlight into an all-purpose search tool instead of just using it to comb your system files, then Flashlight is for you. If you're not interested in code, customizing your Mac, or in tinkering with your programs, then you might just keep looking for another app.
From Nate Parrott:
The missing plugin system for Spotlight. Flashlight is an unofficial Spotlight API that allows you to programmatically process queries and add additional results. It's very rough right now, and a horrendous hack, but a fun proof of concept.