Otzarhachassidus.com
I doubt whether
@Philosophypsych 's grandfather would be able to navigate that site, and I dare say that it's not that easy for non-Israelis and non-techies, the filtering and/or drill down isn't that intuitive.
If you want to reach R' Yoel's shiurim on Tanya, you would want to navigate by ספרים->ספרי אדמו"ר הזקן->תניא and then UNCHECK תניא, expand the section you want to learn (e.g. ליקוטי אמרים, שער היחוד והאמונה, etc.) and check ONLY what you want to see. You can then scroll down in the sidebar, expand the מגיד שיעור section (probably need to scroll up to see R' Yoel's name) and check only the name you want. The filters seem to work in a way of יש בכלל מאתיים מנה, so if you have תניא checked, it will show all shiurim for all sections, to drill down you need to uncheck תניא and check only what you want to see.
If English is the language, then Rabbi Paltiel's InsideChassidus.org is my favorite.
