It's a service funded by the Welsh government, you can send in any drug that's not a steroid, they say you shouldn't send in any drug which you suspect to be illegal but they will still test any illegal drugs you send. The only data they ask for is the postcode area (the first part of a UK postcode) which does not put you at risk given how much of an area each one covers. They also request you fill in data about the drug like if you consumed it and what the effects were, but most are optional to fill in.
It's a one way service, you send them your drugs after filling in one of the forms you can download (which contain no personal information about you), then post the drugs, then they will test them and upload the results to the website. All results are posted publicly here, the site seriously lacks searching features or the ability to link to samples so TripSit has a helpful mirror here which is searchable/linkable.
It shows some seriously interesting results, like the fact that for some strange reason a large amount of the heroin sent in is cut with Fluoxetine, an SSRI antidepressant.
It's much better than a testkit (although requires more effort on your end) as it will show you all of the psychoactive compounds, so if your LSD is a mixture of LSD and an NBOMe, or like this example where Mescaline was mixed with 2C-B-FLY-NBOMe then that will show up, or if your drug contains strange things that a test kit would have no hope of identifying.
The service also extends to generic imports of medical drugs (you can find stuff cheaper than an NHS prescription often). Steroids are only banned from being tested because they had way too many submissions to test and about a third contained no active ingrediant at all.
Edit: I've just found out ACTINOS has been setup in Australia, its been active since December but has only had 9 samples. If you're from Australia I'd spread the news of this to any Australian drug scene sites or even make another post about it on this site. It might lose its funding if it doesn't get more popular.
Thank you for this informative post.
EDIT: Also, please everyone in the UK, use this to help out /r/DNMAvengers!