Diglett's Cave is, quite obviously, filled to the brim with Diglett and the occasional Dugtrio. And really, that's it. Other then the only way to get to back to Pallet Town without Surf it's pointless. At least the Diglett catch will help you against Lt. Surge...
Red, Blue and Yellow: Diglett(95%) and Dugtrio(5%).
Firered and Leafgreen: Diglett(95%) and Dugtrio(5%)
Nuzlocke Warning! In Firered and Leafgreen the Pokemon here have the chance to have the ability Arena Trap. This means you won't be able to flee if your Pokemon is not flying or does not have Levitate as an ability. Also the 5% Dugtrios can be above level 25 so watch out.