Text Generation WebUI (by oobabooga) is the most feature-complete open-source UI for running local language models. It supports a vast range of model formats (GGUF, GPTQ, AWQ, EXL2, and more), backends (llama.cpp, ExLlamaV2, TGI), and extensions including a characters/roleplay system, a notebook mode, and an OpenAI-compatible API. It is the go-to tool for power users and researchers who want maximum control over their local LLM setup.
Free and open-source. Hardware costs only.