Right now, there are four ways to play Delta Green, with a variety of systems:
1) Call of Cthulhu plus the DG Corebook/dual stat version (BRP)
2) D20 Call of Cthulhu plus the dual stat version (d20)
3) Night's Black Agents "Dunwich Sanction" frame plus Trail of Cthulhu (GUMSHOE)
4) NEMESIS system (ORE). There is nothing particularly DG about Nemesis, except that it was written by Dennis Detwiller with DG in mind.
Another way, which might require some work, is to combine Realms of Cthulhu and Agents of Oblivion for Savage Worlds.
Countdown is a mix of articles about DG setting elements and adventures. Two of the former include Russian and British groups analogous to Delta Green. It's huge book with many templates for international characters and psychic powers too, and the later books follow this format somewhat. Most people consider it better than the DG corebook itself.
Eyes Only has two long articles about the Fungi from Yuggoth & The Fate, a shorter article about post 911 gaming, and 3 adventures. Some of this material was released in a hard-to-find chapbook format at the turn of the century. The Artifact Zero adventure has probably the best reveal in all of gaming.
Targets of Opportunity has five long articles about four Mythos threats & a Canadian alternative to Delta Green. While there are no dedicated adventures, the four bad guy groups are easily enough used as campaign starters.
If you want a look at things right away, check out Dennis Detwiller's blog (http://detwillerdesign.squarespace.com/free/), which has four free adventures (Music from a Darkened Room, The Last Equation, The Night Floors & A Victim of the Art - these last two appear in Countdown as well). The NEMESIS rules are there as well. Here (https://dl.dropboxusercontent.com/u/7001919/DG-FuturePerfect1-3.pdf) are the first three parts of his "Future Perfect" campaign. I'm not sure where the final rough-around-the-edges conclusion is.
Delta Green answers two questions which can dog Keepers: "Why would an FBI agent share confidential information with a university professor" and "what the hell has the Mythos been doing since Lovecraft wrote about it?" Its brilliance is answering the latter with modern nuggets of conspiracy theory, like UFOs and the Philadelphia Experiment.