To add mods to your browser-based Eaglercraft experience, follow these steps:
Silence is the enemy of relaxation. The Music Player Mod allows you to play custom audio files (converted to .ogg or resource pack compatible formats) on in-game radios and tape decks.
Still one of the most searched "mods" for survival servers.
Keep in mind that the "hotness" of a mod can depend on personal preferences and the current meta of the game.