Ozone layer -- Saved by getting rid of certain chemicals, although their replacements might be causing global warming.
Acid rain -- Better scrubbers on smokestacks have largely fixed this problem. It turned out that the technology wasn't as costly as it seemed.
Smog in LA -- About an order of magnitude better than when I was a kid, although the cost in poorer miles per gallon must be huge. You may recall that there used to be two different MPG ratings from the government on cars, one for California and one for the rest of the country, with the California one about, I don't know, one-tenth worse. Now, everybody has the California smog-fighting equipment on their cars, so that must increase our oil bill by many billions annually.
Lead -- Here's where one environmental improvement caused another improvement. The catalytic converter (invented by GM and given free to other car companies -- thanks, GM!) would be ruined by leaded gasoline, so unleaded gas was introduced.
Redwoods -- Saved by the Save the Redwoods League, co-founded by Madison Grant.
Pelicans -- Very rare at the beach when I was a kid, now plentiful due to ban on DDT, which makes eggshells brittle
Bald Eagles -- Not plentiful, but they're back. (This is one you occasionally hear about, because people like large vicious animals.)
You might think that environmentalists would promote an image for themselves that says, "Trust us. We fixed problems in the past and we know how to fix them now," but, instead, apocalypse and misanthropy seems to sell a lot better.