Lava Lamp

The glow is small and the texture animates, which makes this good for a lounge, a lab corner or anywhere you want atmosphere rather than actual visibility. What catches people out is housing: the game does not count it as a valid light source. Furnish a room with one and nothing else and your NPC will refuse to move in, and the housing menu will only tell you something is missing, not which piece.

Hang a torch or a lamp somewhere in the room and the lava lamp goes back to being pure decoration.

The Party Girl stocks it, so it is gated behind having her in town rather than behind any boss. Since the 1.4 update it can sit on a table instead of only on the floor, which is where most builds want it anyway.