Edit: This post is obsoleted by this post; please see that one instead.
MIRI are currently holding a donation-matching challenge, until Friday 15th August. You can donate and track its progress by going to the Donations page.
Also, to quote the MIRI Facebook page:
Stellar, a long awaited new cryptocurrency and distributed payment network, made by the founder of Mt. Gox and Ripple, just launched.
You can support MIRI for free by signing up. Every new Stellar user gets 6000 STR, and can send an additional 1000 STR to another user for FREE!
Our Stellar username is “miri”.
This is an awesome opportunity to get in on the ground floor of an exciting and promising new digital currency project while also supporting MIRI.
Registering for Stellar requires just a username and password, no e-mail or verification required. To get the free Stellar, you have to have Facebook Platform turned on (see the first setting on this page) and allow the Stellar App to temporarily integrate with your account. Send 1000 STR to 'miri' or their address (gHhshpzDcfRsie2qxjjHqrsTRe3JSCaUeN), and you will get back the 1000 STR. Once you receive back the 1000 STR, you can then remove the Stellar app like any other in your Facebook settings.
Edit2: The promotion seems more-or-less gone now; consider the part of this post about Stellar irrelevant.
I did this and it took me maybe three minutes (including time to take notes). The Stellar website is self-explanatory. If you encounter difficulties at any point in the process, or if I've forgotten some part of the process, feel free to comment on this post to that effect.
To track follow-up on threads like this: If you donate because of this thread, please indicate that in this comment (you can do so anonymously there if you prefer), and feel free to state that you have donated as a reply to that comment.
(If you're thinking free cryptocurrency is too good to be true, and wondering what the founders get out of it: My understanding is the reason this works is that early investment in a cryptocurrency bolsters it, so that giving away some for free at the start is actually a smart thing to do at first for the founders. No gimmicks or tricks about it asking for card details or anything dodgy like that.)