The "season one" box started things off with standard zombies. They came in walker/fatty/runner varieties, with an abomination too.
Prison /Outbreak introduced "berserk" zombies that needed to be killed in melee as they ignore bullets. They arrived in the same walker/fatty/runner/abomination types.
Toxic City Mall brought us "toxic" zombies, they needed to be killed at range rather than melee, as doing so would release a toxic spray. They came with the same variety as the others.
Rue Morque introduced "Skinner" zombies. Much like standard zombies, but they had a chance of fighting on even after a killing blow, albeit diminished as a "crawler". The walker/fatty/runner varieties remained, with a new abomination joining them.
Angry Neighbours did it a little differently. We got "seeker" zombies, but without the walker/fatty/runner distinctions. They also gave the survivors some human companions to assist in things like searching, building barricades and combat.
Each of those boxed games comes with a campaign, new tiles, and extra rules to fold in with the new locations they bring. Lots of new equipment cards, in addition to the extra zombie spawn cards. So many of each, by the end I the stack of either was over twice the height of how things started, and had to be split up to stop them toppling over.
There's also been some smaller boxes of extra stuff. A few are just more of the zombie types from the boxed games. However, there's also boxes of crows, and dogs, and VIP zombies. The animals are really just scarily fast, but the VIP are models of iconic things like Santa, Elvis, and Pimps (10 different things I think over the two boxes) and killing them lets you pick up new and fancier weapons.
A box of dog companions also exists the help the survivors.
