Something that I gather people have been doing for as long as there has been roleplaying but that I've only picked up semi-recently is the inclusion of a subtable of unique entries for encounters. The first I saw of it was over at Middenmurk, and I've been studiously including them ever since. Why? Well, they represent an added level of flavor as well as the possibility for what –C calls "red herrings," new adventure hooks, and myriad layers of surprises and scenarios which the players can use to their advantage (or bungle and get used by, howsoever the case may be).
I suppose first of all I should be clear what I mean by unique entries. These are one-time encounters that are removed from the table after they are experienced. Thus a main encounter table might draw to a subtable called "Local Flavor," which includes...
1. Three goblins picking over the remains of a halfling slain on the road
2. Two men swinging as thieves from a nearby bough
3. A rainstorm (do not remove)
4. 2d6 royal messengers fleeing from 2d8+4 orcs
These events (other than the rainstorm) do not happen often enough in the lifetime of a single character, or perhaps even multiple characters, to merit a full entry on an encounter table. However, they are interesting and conceivably players should be able to stumble upon scenes of this nature. Well, you say, why not just show them the scene when you are ready? Because that robs me of my fun, as well as the opportunity to let the world feel like its natural and organic. If players don't see everything when they play (ie, if I don't unfurl my planned scenes like a carpet) they will be much more excited about the things they do see. I know this from the rare entries in my encounter charts. Only one group ever encountered a Cloud Giant bard wandering the hills looking for tales... but it was one of the most memorable encounters of all time, because it didn't happen very often. So it goes with unique encounters, but even more so.
Having used them for a little while now, I can heartily say that they provide great roleplaying opportunities as well as new tools with which to think about the dungeon/city/whatever. Like everything that happens, they can serve as adventure hooks if the PCs are interested in finding out more about them, they add real flavor and the sense of a living environment, and they really aren't that hard to make. I'm sure this is old news to everyone, but for me, it is a startling revelation of immense proportions. I suppose that's what I get for never having read Dragon or really purchased modules or anything: the best tricks I'm left to stumble on myself!