Yahoo! used to have a large free e-card service, but now,it has partnered with American Greetings. The website has a number of Mother's Day cards, but none of them are free; instead, you can sign up for a Free Trial. Since it's a Free Trial, you could sign up, then simply dump the Trial, but it's simpler to just use one a free, full service (later).
In case you're wondering, American Greetings has the same sort of set-up. You can sign up for a Free Trial and you can then send an e-card.
Blue Mountain, which used to be a large free e-card site, has gone the way of paid. Just as with Yahoo!, you have to join to send an e-card. It appears you can get a one-week Free Trial, however.
Another site you can take a look at is egreetings.com. Here you can find a number of free e-cards, with no trial or subscription required.
Regards.com also has a selection of free e-cards. Their selection isn't particularly large, however. Be sure to select from among the free e-cards are there are many that are paid, as well.
One of the largest e-cafd sites that still, after all this time, continues to focus on free e-cards is 123 Greetings. Prominent on today's front page is an image of what appears to be a mother and two daughters pointing you to the section for Mother's Day e-cards.
That section of the site does rotate, but it's easy to find links to the Mother's Day section. In the "Popular Now" area of the front page, 8 out of 10 of the cards are Mother's Day related, at least right now.
There is also Some Ecards (someecards.com). These are free, as well, but these are snarky and meant to be humorous. An example: "Thanks for always thinking about me to the detriment of your own health."
Finally, there is the site original-cards.com. These are generally quite simple cards, but they are free. One other notable thing is that that site has a section full of "naughty" e-cards, as well.
It's still not too late. Visit one of these websites and send a free e-card to a mother you know, whether it's your mother or not.