Direct 2 Drive is a web based online distribution platform that has no client side program involved.
Because of the lack of a client side application things are a little different. It does mean there are more copy protection codes involved and there is a browser plugin required for the downloading of the setup files.
Aside from this though you only have to watch the site for a couple of weeks to realise that they often have some fantastic deals running almost constantly. They also have plenty of their own exclusives compared to other popular systems.
Again it's useful to keep up to date with the site because of the deals and price comparisons. And if you don't want to install client side applications it's the obvious favourite.
Advantages:
Web based rather than Application based
Good bundle deals and sometimes best prices
Large and alternative catalogue
You can buy comics/game guides too!
Some independent developers given a chance here
Disadvantages:
Web based rather than Application based
Added copy protection for each title
It really is a nice alternative to the other systems, the fact that you can buy guides and comics is a big plus.