<p>I'm working on deploying a small community site. User registration requires nothing more than a username, email address, and password. I'm not even asking for a name, and certainly not storing any sensitive data.</p>

<p>Should I still invest in an SSL certificate? Would it be considered terrible practice to transmit a user's password without one?</p>

<p>This is just a personal project, so I'd like to avoid the extra cost if I could, but I can't help but feel I'd be irresponsible if I didn't secure everything properly.</p>

I wouldn't bother with SSL for something like that.
Think about it... there are a million messageboards on the Internet
and none of them use SSL.
Unless you are storing Credit Card numbers or other sensitive
financial/personal information, I just don't think it's worth the
cost.
EDIT: This response is totally outdated. Yes, EVERYONE needs SSL.  Most hosting companies include support for Let's Encrypt (free SSL) so you don't have any reason not to use it anymore.