Indie puzzle game SpaceChem is 50% on http://store.zachtronicsindustries.com/
You can buy it paying in $ and it is $7.5 (even though the website states it's $8!). It turns out to be £4.59 (provided you don't have change commission from your bank)
Once purchased you can redeem the game on Steam and download it from there, or just download it from zachtronicsindustries.com and install without Steam.
You can also purchase it directly from Steam and it's 7.5 euros (could someone check the pound price please?)
The games is really good (EDGE: 9/10) and you can buy once and install on Windows, MacOs and Linux.