This is a good question: maybe we should put together one master list somewhere, indexed and with descriptions?
To those that others have already mentioned, I wish to add:
Mordechai Torczyner's WebShas: an index to the Shas Bavli that he has produced (and is still producing). To give you an idea of what it's like, this is the page for activities prohibited on Shabbat;
Rav Nissan Kaplan's collected shiurim. Perhaps not for everybody (though I love it, and it helps me feel like I'm still in yeshiva on days when I wish that I were). His mussar schmuessen are my favourites, though I'm also a long-time fan of his halakha shiurim. His gemara shiurim are still a little too advanced for me, but they come with PDFs of the material that he covers in point form, and they're something that I'm working towards. Rav Kaplan is a mashgiach ruchani at the Mir Yeshiva, Jerusalem;
Somebody has already mentioned the DAF, but I want to emphasise it in case people here are not familiar with it. This page constitutes their page-by-page summary of Masekhet Eruvin, and if you fiddle around with the site you'll find all sorts of other good things;
A friend and old yeshiva colleague of mine, together with some friends of his, has put together a truly excellent daf yomi application called The Mercava. This has a tremendous amount of potential;
Finally, in addition to using Jastrow's dictionary (which I'm told can be found here), I rely very heavily on Morfix, which is an online Israeli Hebrew dictionary. It's fast, it's reasonably accurate, and if you're working through anything in Israeli Hebrew (for example, Kehati's Mishna commentary, or Rav Steinsaltz's Hebrew peirush on gemara) it's hard to do without.
So far as blogs are concerned, there are really not many that I like, but the absolute top of the list would be On The Main Line, which is a very scholarly (and very entertaining) exploration of Jewish historical esoterica!
Second to that, and at times a distant second, is Rabbi Gil Student's Torah Musings. It can be a very good source of Jewish news, and Rabbi Ari Enkin's halakhic contributions are always worth reading. Unfortunately, they do not yet seem to be archived anywhere on the site, and so you have to look through manually in order to find them.
I also really enjoy occasional posts at The Talmud Blog, find very useful Rabbi Natan Slifkin's database of sources testifying to the scientific errors of Chazal, and sometimes enjoy his blog, Rationalist Judaism, as well.