The classical canon gets good coverage with suttacentral and accesstoinsight, but I was wondering if there was such a website for Mahayana writings. (Post-canon writings diverge and become extensive, meaning different translators, copyrights etc, but if something relatively comprehensive-ish is around, would be great!)
I use Fodian.net a lot.
There are two categories of Mahayana texts, the ones that made it to China and the ones that made it to Tibet.
Within a specific sect or tradition, some sects have fantastic online documents, such as SGI's Nichiren library.
I'm less familiar with the Tibetan ones, but I do know that some of the Mahayana texts also made it to Tibet.
Scholars like Jan Nattier sometimes has to do comparison of the Sanskrit, Chinese and Tibetan to reconstruct a text.
I love http://chancenter.org/
It has an extensive collection of free e-books, Chan Magazine & Dharma talks (in texts, audio, video) by highly qualified Zen Master Sheng Yen and other ancient and modern teachers, such as Hanshan Deqing, Xu Yun etc.
The Dharma talks and the books cover full specter of Mahayana teachings, from basic topics like Three Refuges to the practice of Six Paramitas, to intricacies of Shurangama sutra and so on.
All the instructions are very practical, clear and inspiring.
More than 1000 of brief videos by Master Sheng Yen on various questions about Dharma: https://www.youtube.com/user/DDMTV05 (English subs. You can find also versions with Spanish subs and maybe some others).
You can try to take a look at Bukkyo Dendo Kyokai. They are trying to translate most of the tripitakas into english but right now they only have a few text translated.
A Biography of Sakyamuni
The Lotus Sutra (Second Revised Edition)
The Sutra of Queen Śrīmālā of the Lion's Roar
The Larger Sutra on Amitāyus
The Sutra on Contemplation of Amitāyus
The Smaller Sutra on Amitāyus
The Bequeathed Teaching Sutra
The Vimalakīrti Sutra
The Ullambana Sutra
The Sutra of Forty-two Sections
The Sutra of Perfect Enlightenment
The Vairocanābhisaṃbodhi Sutra
The Platform Sutra of the Sixth Patriarch
The Baizhang Zen Monastic Regulations
Shōbōgenzō: The True Dharma-Eye Treasury vol.1
Shōbōgenzō: The True Dharma-Eye Treasury vol.2
Shōbōgenzō: The True Dharma-Eye Treasury vol.3
Shōbōgenzō: The True Dharma-Eye Treasury vol.4
Tannishō: Passages Deploring Deviations of Faith
Rennyo Shōnin Ofumi: The Letters of Rennyo
The Sutra on the Profundity of Filial Love