Standard: I jot on scrap paper or on a tablet draw file the initiative sequence with number, name, then cross off as initiatives are eliminated.
D&D running AL...
I often have used a card deck of custom cards - I write the name, class, level, initiative mod, dex. Their laminated, so I use wet erase markers (Staedtler gray barreled pens). The cards also have a hole punched. I feed them on a ring when initiative is set.
Sentinel Comics: Since initiative is popcorn, I make a tracker with PC seats, and put a marker over the PC or villain spot when they have gone. When playing via discord, the bot handles tracking that.
FFG Star Wars: I use the tracks from WFRP 3E... Red for NPC, Green for PC.