Apparently it infers a username from your submitted email address. However, the username key is set as unique and errors out, dumping its debug info on the page:
<class 'psycopg2.IntegrityError'> at /register
duplicate key value violates unique constraint "rcpts_user_user_login_key" DETAIL: Key (user_login)=(receiptapp) already exists.
Python /usr/local/lib/python2.7/dist-packages/web/db.py in _db_execute, line 586
Web POST https://receiptapp.org/register
I've been doing this for 7 years with great results and love the zero configuration needed for registering at any site with a unique email address.
In the past few months, however, I've been getting spam that simply brute-forces usernames across my entire domain, leading to hundreds and thousands of spam emails per day. Gmail catches all of it but this makes looking for the occasional false positive very difficult. I've lost a number of emails that ended up in my spam folder between pages and pages of real spam.
I never got used to Chrome's Developer Tools. Firebug and a properly working Scrollbar Anywhere / Grab-n-drag (lets you right click anywhere and drag up and down to scroll anywhere in the page) are the only extensions keeping me on Firefox.
The equivalents on Chrome are simply not good enough. I don't know if this is an issue with all Chrome extensions, but Wet Banana / Scrollbar Anywhere on it seem to be running in a low priority and thus have sub-par performance. Scrolling is choppy and doesn't always even initiate, depending on where on the page I right-clicked.