I've become a fan of what I like to call the stacking table - designed so rolling dice of different sizes will give you a varying range of possibilities. I'm not sure if it has another name. I first encountered the concept in A Red & Pleasant Land, where it's used both for the instant location tables and the encounter table on the map in the endpapers.
Stacking tables are useful when you need to sort entries into different classes that only become available in certain situations. For example, the random encounter table for my LotFP Balkan horror campaign was inspired by the table on the R&PL map:
Roll d4 on a road, d8 in the wilderness, and d12 at night:
1. Ottomans
2. rebels
3. gypsies
4. bandits
5. stag
6. boar
7. bear
8. crows
9. bats
10. wolves
11. spirits
12. elves/dwarves/hobbits
A random NPC table would also work well as a stacking table - stacked by how civilized the surrounding area is. Here's an example for a generic medieval fantasy setting:
Roll d4 for the wilderness and up to d20 for a city:
1. hunter
2. trapper
3. merchant/gypsy
4. farmer
5. soldier/spy
6. noble
7. blacksmith
8. innkeeper
9. priest
10. midwife
11. guard captain
12. mayor
13. jeweler
14. weapon smith
15. cheesemaker
16. gang member
17. philosopher/astronomer
18. artist
19. high priest
20. ruler
And here's a bonus example for a post-apocalyptic setting:
1. scrap scavenger
2. hunter/falconer
3. raider
4. hermit
5. prospector
6. trader
7. water diviner
8. cannibal
9. slaver
10. cultist
11. mercenary
12. herder
13. moisture farmer
14. salt miner
15. fungus gardener
16. mechanic
17. barkeeper
18. sheriff
19. boss
20. messiah
