The Forgotten Realms has long been my favorite campaign setting. In addition to having a rich selection of gaming material, there have been many, many novels set in the Forgotten Realms. Some of the most popular computer games have been set in the Realms, including (Baldur's Gate I & II, Icewind Dale, Pool of Radiance and the older SSI computer games).
The setting is one of the most popular of all the D&D settings; Wizards of the Coast run a website specifically for the Forgotten Realms. With the release of the fourth edition of Dungeons & Dragons, Wizards of the Coast has made some drastic changes to the Forgotten Realms campaign setting - changes I don't like at all. At this point I'm still playing using the 3.5 edition ruleset, and will likely continue to use the campaign setting without the changes introduced by the 4th edition revamp.
If you're new to the Realms, have a peek at the brief Introduction to Faerûn page I've written up.
Updates: I've added a Downloads page, and moved the Espruar and Dethek fonts to it. I've also added the 3.5 rules for the Sha'ir class originally from the Al-Qadim setting. Over time I'll try to add more information and reorganize a little better.