Cline-pass is now serving as my primary open source LLM API provider, but it may be worth pointing out that I still use OpenRouter constantly for all sorts of tasks. Its killer features are: support for hundreds of well known models (commercial and open source), and very importantly, built-in support for the OpenRouter API in just about every AI environment that needs LLM access.
I use it constantly to try out new models - https://openrouter.ai/models is my favorite daily news source about new models, generally the instant they appear.
I use it to test the capability of various models in new applications I develop, and to get models hooked up in new agents & software that I'm testing from other developers.
OpenRouter will also be trusted as a backup for any other LLM API provider I hook up.
OpenRouter is also typically one of the quickest APIs to set up when I want to get a client going with a new app. Most of my clients have already learned how to use it, so they often don't even need my help. The instructions 'just select OpenRouter as the provider and paste in your API key' becomes familiar to everyone.
OpenRouter also has a fantastic reporting system, so it's great for testing cost comparisons between LLMs, for tracking cache hits, and to get all sorts of other feedback about model performance.
Other features like LLM Fusion are also useful.