Me First and The Gimme Gimmes do amazing ska covers of popular music.

Straight up ska (in addition to what other people have said): Streetlight Manifesto, Mad Caddies, The Interupters, Big D and the Kids Table, The Specials.

If you wanna go old school and to a couple of biggest influences on modern ska and way more punk: Choking Victim (later morphed into Leftover Crack) and Operation Ivy (morphed into Rancid).

Roots (like ska but a bit more chill): Stick Figure, The Expendables, Rebelution, Pacific Dub.

You may also like Sublime and the band made after that Sublime with Rome (although personally I don’t).

Also something not quite ska but kinda in the vein of, shameless plug for a great New Zealand band called Fly My Pretties.