Here are some of the resources that I find myself using on a routine basis.
System Agnostic Resources
- Anydice: You tell Anydice what dice you'd like to roll and it tells you what the probabilities of each result will be. I use this site a LOT for everything from helping my brother and wife optimize their 4e characters to figuring out what the best dice to roll so that the probability curve of a random encounter table matches what I'd like it to be. The site can also compare several different dice expressions if you write several different lines of dice roll output (so, if you can compare the probability distributions of 1d3+1d6 to 1d4+1d5 and 1d8+1).
- Hexographer: Free hexmap software! Yes, there is a paid version, but as far as I can tell, the free version has everything you need to build a flexible hexcrawl sandbox.
- Wizardawn isn't so much system agnostic as it is supportive of multiple systems. Need a table or a map or, fuck it, a whole night's worth of adventure? Wizardawn, ladies and gents, Wizardawn. Plus, it has an amazingly kickass name.
- While we're on the subject of online generators and stuff, I'd be remiss if I left out Mithril & Mages. Like Wizardawn, it supports many different systems (well, a few different systems) rather than no-system. It does, however, have plenty of system agnostic useful stuff there, too.
Dungeon Crawl Classics
- Purple Sorcerer Games Tools: Purple Sorcerer has made a suite of terribly useful tools for DCC including a "funnel generator" (generates 4 level 0 characters for your next funnel game; I've been toying with the idea of just printing out a bunch of these on cardstock so I can have a deck of level 0s available whenever someone is looking to refill their "hand" of characters), a character generator (for level 1 or higher PCs) and links to a dice roller app and an app called the Crawler's Companion which handles and indexes much of the crazy info (corruption rolls, critical charts, spellcasting charts, etc.) that the game requires. Great stuff that I use at the game table every session.
- Jeremy Deram's DCC Resources Page: Jeremy Deram of People Them With Monsters has written an excellent series of materials for DCC, many of which I use in the Game of Taps. His DCC RPG Ref Sheets (a worthy successor to Ready Ref Sheets) is quite frankly awesome and I've made a copy for each of my regular players. Plus, a ninja class because fuck you.
Online Gaming Stuff
- Roll20 is the VTT of choice for yours truly and a growing portion of the Google+ community.
- Devin Night's Token Site features tons of great-looking tokens for VTTs. Devin ran a Kickstarter in August of 2012 that raised almost double the target funds! Great top-down tokens.
- Map of Metal.com: Holy crap is this site awesome for exploring the history of metal!