I would not be where I am without my involvement in OSS or other projects, mostly due to the gained experience. The "trick" about putting it on your resume is,

I would not be where I am without my involvement in OSS or other projects, mostly due to the gained experience. The "trick" about putting it on your resume is, if it's significant, to make it not sound or placed any less worthy than any other experience on there (possibly also avoiding terms like "free", "personal", etc)