The two collections, marked US and UK respectively (but without any restrictions on the ability to download them), offer a sampling of the 2000AD/Rebellion Universe.
The US edition (available here) features Dredd reprints, with stories by John Wagner with art by Ian Gibson and Pete Doherty. There's also a Ten Seconders story, along with Zombo, and Button Man. I only scanned the issues presented, but they appear to all be one-shot stories designed to get the reader intrigued enough to look for more, which is exactly what a sampler should do.
In the UK edition (available here) takes a different tack, with a lead story featuring Defoe, by Pat Mills and Leigh Gallagher. That looks really cool, and I can't wait to sit down and read it in more detail. The rest of the sampler features Cradlegrave and Insurrection, the latter a tie-in to the Dredd universe written by the D part of the DnA team, Dan Abnett. It's a bit shorter than the US edition, but man, if you're complaining about the length of free, please come here so I can hit you over the head with something.
I'm still moving into reading comics from the 2000AD Universe, filling in a gap in my comics reading that I should have taken care of years ago. If you've never tried any of their offerings before, this is a great opportunity.