Like Steam and GOG, Itch.io has developed a program to manage your library, install and launch your games. It's available for both Windows, Mac and Linux. The open source crowd will be pleased that all the code stuff is available to play with (in JavaScript). It's currently in alpha but as it's quite simple in features, there shouldn't be a lot of problems (excepting there are no guidelines for game packaging for devs)

It's available here:
http://itch.io/app

The github directory:
https://github.com/itchio/itch

The issue tracker and "official" forum thread:
https://github.com/itchio/itch/issues
http://itch.io/t/11307/the-itchio-desktop-client-is-here

What is Itch.io?

A cucumber?

http://itch.io/

Yay, more client software, just can't have enough client software...

So is this optional, like gog, or not optional, like gamersgate? I've downloaded a few things from there but never opened an account. Not planning on it at the time being. But never planning on it if they took the gg route.

I get it.. but please stop with the clients.

Taken straight from the blog post:
"The itch.io website is here to stay, we’re not planning on replacing it with a desktop app."
http://blog.itch.io/post/135196264464/say-hello-to-the-itchio-app-itch

And with the neo-hippie mentality of the website, I highly doubt they will ever go the "app only" road....

EDIT:
re-confirmed on their Twitter:
https://twitter.com/itchio/status/677550436837154816

Yeah, I kinda got that feeling from my brief visits. But thanks for the confirmation.

I suppose you wanted to write "all those clients" (plural)...

At its core, such a program would be just a UI which launches the .exe file of a specific client and do some "auto-fill" for username and password. The problem is that to do the auto-fill, you would need the source code of the app or do some reverse-engineering (as there is no "standards" like web forms in HTML) and legally you are not allowed to do modifications of the client programs of Steam and GOG.

Actually connecting all the clients is probably not going to happen, but I've found that Launchbox (https://www.launchbox-app.com/) is extremely handy at being a frontend for everything. It'll run Steam, GOG, "unaffiliated" PC games, emulators...

Actually that looks pretty damn good, thank you Andy!

Well, I wasn't aware of that. Whats the deal on it, can it run totally offline? I realise that it would connect to a database to download pictures of the games and metadata, but is that then stored offline, i.e. can it be used like Calibre, just like a library which you can request data.

Yes, the metadata stuff, once downloaded, is kept on your PC in the Launchbox folder. It really is just a nice, very user-friendly frontend that allows you to organize your games by platform (I believe it began as a strictly DOSBox frontend that just kept expanding). Most of the work is just pointing it to wherever a game's application file is on your PC, and telling it which emulator or configuration you want it to use.

I'm still using the free version, btw. Although I'll likely upgrade to the paid version eventually, the free version is fully functional and merely lacks some bells and whistles like the living room/"Big Box" mode.

Awesome. I probably won't use it, but nice to see that their growth continues at a high pace. Not bad for a storefront that lets the dev/publisher decide what Itch's cut of every sale is (they can even set it to 0).