GOG have kicked off their DRM-Free Summer Sale for 2015 today:

It’s that time of the year again.

Temperatures are rising, summer break is almost upon us, and whether you’re going for a classic staycation or taking your laptop for a trip – we’ve got what it takes to make your summer vacation more gamey and 100% yours. Today, we’re inviting you to a truly exhilarating, DRM-free summer adventure!

We’re starting with a splash and diving straight into 100+ titles on promo in twice-daily deals, deep-discount bundles, and with FREE games to unlock! With new deals appearing on the site every 12 hours, we’ll have more than 100 offers every day up to 90% off. We also have a few fun surprises in store to keep things fresh and worth the regular trip back to the summer sale!

Every summer adventure needs something special, that’s why this year you can earn up to three games for free during the sale! Just do your thing, and the more you spend, the more free stuff you’ll get! Regardless of whether you own the game or not, you’ll receive a giftable code via email so you can redeem it yourself – or share it with a friend. The rewards — free for everyone whose total spendings during the Summer Sale exceed a certain amount — will include:

SimCity 2000 – free for everyone who spends at least $1.
S.T.A.L.K.E.R.: Clear Sky – free for everyone who spends at least $20.
Xenonauts – free for everyone who spends at least $50.

Plenty more summertime joy awaits us all, including a surprise giveaway, exciting game premieres, and more! The 2015 DRM-free Summer Promo is set to last 18 days, from June 3 until June 21 at 11:59 AM GMT (13:59 CET / 4:59 AM PDT / 7:59 AM EDT).

There aren’t any Ultima games or bundles on sale yet, but it’s almost a certainty that at some point the series will enjoy a significant discount.