As Mr G said above, you can create a Handout, and assign the permissions of that Handout to "All Players" (for the "In Players Journals" setting). In the Description box, you can enter a link (e.g. "<a href="http://google.com/" rel="nofollow">http://google.com/</a>") and it will turn it in to a clickable link
Yeah, I've been doing this. Venus D., here is an example of a clickable link in a Roll20 Handout. It leads to a PDF character sheet:
Also, since the length of the URL matters here (nobody wants a too long "ugly" text), I think that it's a good idea to shorten it by using <a href="http://tinyurl.com/" rel="nofollow">http://tinyurl.com/</a>