Is the phrase "make waves" used with the sense "create a snowball effect"?
Perhaps you've got [have] a ripple effect at the back of your mind!
a ripple effect
if something has a ripple effect, it affects something else, which then affects other things
Court rulings often have a ripple effect, spreading into areas of law that weren't part of the original cases.
You'd want 'Let's create / produce a ripple effect!'
"Make waves" is a well-known English idiom, and it can have several meanings, but I don't think it fits the described context:
make waves: to cause problems by making suggestions or criticisms
(Macmillan English Dictionary)
makes waves: to disturb the status quo
(Webster's Unabridged)
make waves: create a significant impression
he has already made waves as a sculptor
(Oxford Dictionaries)
UPDATE: At the moment I cannot think of a suitable idiom for your intended meaning. I think Edwin's "ripple effect" captures one aspect of it, but I'm not sure "let's create a ripple effect" sounds like a good motivational line, which seems to be important.
You mentioned the end result of the open-source community effort would be "something big" (i.e. a popular, successful open source project or initiative.)
Let me suggest a different water-related idiom that basically means "make famous" or "attract attention": make a splash.
attract a great deal of attention
(Oxford Dictionary)
Take a look at the open source rookies that made a splash in the past year
(infoworld.com)
If this fits your purpose, you could end the post with "Let's make a splash together!"