Tools? I don't know of any website that is good for this. Must say that one of my future projects (about a month away) is a website where people who are looking for co-founders can post messages and interact. Its weird that I must build this tools.
Though I must admit that finding a co-founder is not as easy as posting an ad on a website. A co-founder is akin to a business wife/husband. You just cant go and marry anyone. There has to be a connection between the two people. Maybe not be best friends, but a common understanding of what the big goal is. Skills must also be somewhat complimentary. I, for one, look for people who have business/management experience and will get things done. I'll take care of the software, they will take care of the management, but we will both (or all) have a say in all things. Maybe the other person might not know an SQL Join from a Python variable, but they can do testing, design feedback, Q&A, R&D, etc. I might not be a good business manager, but I can call people on the phone and get appointments, help with a presentation (or do it myself), help with the marketing, etc.
Its not a what you can do versus what I can do, but a what we can do together. Like a marriage.