I have an open-source project that is quite popular with dozens of downloads per day. I use logos from big companies in there (e.g. Cisco or Microsoft) to customize some web pages. The logos are important for the project's effectiveness.
Can I use these logos under "fair use" considering that I'm not making any money out of the open-source project? It's worth mentioning that I'm confident that users of the project are using it for commercial uses (e.g. to sell their services).