For those that are happy with GOG keys, these games are roughly comparable in price to recent Steam sales.

All GOG keys:
Undertale £3.49
Darkest Dungeon £9.49
Firewatch £8.19
FTL: Advanced Edition £1.99
Transistor PC £3.79
The Witcher 1 (£1.19) and 2 (£2.39) are also available
System Shock 2 £1.19 and others

Undertale Metacritic: 92% (43 critics, 7.9 from 3412 users), Steam: Overwhelmingly Positive 96% (76093 reviews)
Number 1 (non-violent) game of 2015 for TotalBiscuit https://youtu.be/nNmNqnJcRCI?t=2068 -

Darkest Dungeon Metacritic: 84% (55 critics, 7.8 from 381 users), Steam: Very Positive 87% (21130 reviews)
It won RPG of the year 2016 on the PC Gamer (US) site: http://www.pcgamer.com/best-rpg-2016-darkest-dungeon/ ( http://www.pcgamer.com/game-of-the-year-awards-2016/ )
TotalBiscuit likes it: WTF Is... - Darkest Dungeon ? and put it as his number 7 game of 2016

Firewatch Metacritic: 81% (65 critics, 7.3 from 818 users), Steam: Very Positive 87% (23048 reviews)
Totalbiscuit's take: WTF Is... - Firewatch ?

FTL: Advanced Edition Metacritic: 84% (42 critics, 8.4 from 1325 users), Steam: Overwhelmingly Positive 97% (34056 reviews)
TotalBiscuit's review was complimentary: WTF Is... - FTL - Faster Than Light ?, he liked it so much he put it as his 3rd best game of 2012.

Transistor Metacritic: 83% (35 critics, 8.3 from 682 users), Steam: Overwhelmingly Positive 95% (16632 reviews)
Totalbiscuit likes it: WTF Is... - Transistor ?

I created this deal after it was pointed out by Dawsy that the multiple deals for the individual games I made seemed spammy.

I also liked Superhot at £8.59, but you can see how that went (apparently it can be bought more cheaply!):
http://www.hotukdeals.com/deals/gog-superhot-pc-8-59-gog-com-2620356