If Deus Ex is one of those series that you've always said you'll play some day but you've never actually got round to buying the games, then today is as good a chance as any. The original and the follow-up, Invisible War, are both on sale for less than $1, while 2011's Deus Ex: Human Revolution is $3/2. That's a lot of game for not much money.
The most recent in the series, Mankind Divided—which split long-time fans—also gets a permanent 50% price cut, which makes it $29.99/£19.99. I wouldn't buy it at that price, though, because it's on sale for less than half of that fairly regularly.
So, where should you start? The obvious answer is 'at the beginning', but if you're new to the series, then Human Revolution is more accessible than the original, and almost as good. It's polished, cinematic, packed with detail and gives you plenty of choice about how to approach its missions. Tom's review (94%, no less) is here.
Of course, the original Deus Ex is a good bet if you can get past the dated visuals. As Andy wrote when he revisited the game last year, it still holds up, and its huge, dense levels are a joy to explore.
Browse through the sales here. They end tomorrow.