There are numerous ways to organise academic meetings, large and small. The most critical matter is having a clear aim and a theme that brings people together in new ways - not just to give presentations but also to make personal academic connections. For this reason, the social aspects of a meeting (workshop, seminar, conference, or congress) can be just as important as the academic aspects.

I have not used any of the systems below, so would welcome comments by anyone who has!

EasyChair - describes itself as "probably the most commonly used conference management system". A free service is offered, that looks useful, and then more advanced systems that are priced according to the maximum number of submissions that the organisers intend to consider. Overall, the costs appears to be about 50 cents to one dollar (USD) per submission.

Open Science Framework for Meetings -  The Open Science Framework is creating open source tools to support 'the entire research cycle'. This includes tools for scientific collaboration, writing, project management, and meeting management.  

Their mission statements include the following: ' The OSF is a free, open source service of the  Center for Open Science . We’re aligning scientific practices with scientific values by improving openness, integrity and reproducibility of research.'