Neilk40: Well Black Mirror 2+3 are one story, Black Mirror 1 is like a prequel to that story, so you could start at 2 then go onto 3 as the ending of 2 leads straight into the 3rd one.
Yes you can download via browser, file size isn't that big so it won't take long at all especially with a fast connection. not sure if it's DRM free or not(think someone said it uses gameshield but that might just be the trial version), i have an original boxed copy, haven't got a DD version yet.
Not sure about 2 and 3, but that's the deal with the original Black Mirror. No client needed, you download the "trial", you unlock it with a serial key. Pretty straightforward, although I don't think they store that for you anywhere, you get the key and links in the mail, so better keep that safe for future installs. Aura is the same way there, but Darkfall: Lights Out DC is DRM-free, so I guess it varies.
For any Myst fans (or first person adventure fans):
RHEM Trilogy Schizm II - Chameleon Aura 1 - Fate of the Ages Aura 2 - The Sacred Rings All $2.49 each over there. For that cheap-ass price there's also
Dead Reefs,
Realms of Illusion - Sentinel and [url=http://Traitors Gate 2 - Cypher]Traitors Gate 2 - Cypher[/url].
At $4.99 I'll echo Witchers recommendation for
and add [url=http://www.justadventure.com/shop/26/the-experiment]The Experiment (aka eXperience 112, you know, that French game where You don't control the main character but the computer system of the place where's she's trapped in, yeah, that).
There's also
The Dark Eye - Chains of Satinav for $9.99
