Haven't posted in a while, here's a thing. Crossposted from a 1e group on the Book of Faces:
I dig my little subsystems, and random encounters are no exception - randos are great because I get to be surprised by the events that transpire in play, same as the players are. Here's my formula(e).
For wilderness encounters, I use the check frequency and dice described in the DMG (frequency depending on terrain, die type depending on proximity to "civilizing influences," and 25% of encounters in patrolled areas "converted" to a patrol encounter), but I roll randomly (on my trusty d24) for the actual hour when any encounters occur. In dungeons, I'll default to the "1 in 6, every two turns" frequency from Basic, more or less depending on the location.
In addition to the normal "Monster Manual encounter" on a result of "1," I have further possibilities for encounters on the die - I don't have the actual table in front of me, but it's something like "hunting/animal encounters on a 2, "interesting flora" on a 3, noteworthy natural scenery on a 4, trail/wagon/animal "incidents" on a 5, a new "Ravaged Ruins" result from the Wilderlands tables on a 6, so on and so forth. (These really help the druids and rangers come to the fore in wilderness travel, I find.) Plant and animal encounters rolled from CDD#4 Old School Encounters Reference.
Encounters on roads are a special case - I roll an extra chance every hour (1 in d6 on an actual Roman-style road, 1 in d10 on a trail) for a "road encounter" (I use the list from CDD#4, but there's a great list in Dragon #105, published as "Travel Works Both Ways"), in addition to the normal wilderness roll.
On top of all this, for ANY encounter, once I've determined one is occurring, I roll the same chance to see if there's already an encounter with another party (monster, NPCs, etc.) in progress. If so, I'll roll some quick reaction rolls to see whether it's a fight, ongoing negotiation, or what. (Note that on the wilderness rolls, with the expanded stuff I'm using, this could totally end up being "ogres picking daisies for pret-ty ogress" or something, haha.) This extra roll can result in some really interesting situations, for what it's worth, I recommend you try it in your game - the "situation in progress" stuff can be a great opportunity for interesting roleplaying.
Finally, if I'm not using an existing local table, I'll throw a quick d6 to decide which book I'm rolling out of (1-2 is using the table from the DMG, 3-4 the tables from Fiend Folio, and 5-6 the tables from MMII), and, generally speaking, if the first result doesn't really blow my skirt up I'll do a quick scan of my notes/maps to see if there's a "locally-sourced" encounter to use in its place. (This is where those monster grudge matches come up - "Soooo, remember last time you came this way, and you webbed the hill giant chieftain in place while the thief braided his toe hair? Well, so does he.")