3.5E SRD. This has turned into my SRD bible, if you will.
Creative Mountain Game's latest is 30 Things Can Happen! It's an assortment of random tables to add flavor to your adventures as they are happening. They add the little things that bring life and spontaneity to the world your party is adventuring in.
You get 30 tables broken down by category: While in a City, While in the Country and While Underground. Each table is further broken down by thirds. So, depending on how you look at it, you have 30 tables with 30 entries each, or you have 90 subtables with 10 entires each. Priced at $4.50, you are effectively paying 15 cents a table (or 5 cents a subtable). A fair price in my humble opinion.
30 Things Can Happen! is another excellent resource for my upcoming sandbox ACKS campaign. I guess you could use in it a 4e game, but that would require one to take a break from the endless combat ;)
From the blurb:
Never be at a loss again with these thirty tables of random Medieval Fantasy events and findings in urban, rural, and underground locations. Each thirty entry table is divided into three ten entry categories, some with additional sub-options, making for nearly one thousand random results including the fun and fantastical. Use these pages to help flesh out locations during prep or to perk up a flagging game session. This invaluable resource is system-free and ready to help with any fantasy roleplaying game.