I have a thing for 20-sided dice. I've always thought they were neat, even when I was a little kid who thought D&D was satanic. I've always wanted to find ways to use them in other games. I wonder how much it would change Monopoly to use a d20? You'd get around the board faster, but you'd also have a chance to roll a 1 (which is impossible in normal Monopoly).
I started collecting RPG dice about a year before I actively started looking for a D&D campaign. Here's some pics of my collection:
I know it's silly, but I like to match up dice with the personality of the character I'm playing. A magic user might get something clear or sparkly, while a melee type might get darker colors. I also like to get matching dice bags if possible.
I also like collecting weird dice dice, in strange sizes or that show funny results. Clockwise from top left: Large dice, glow-in-the-dark dice, tiny dice, and clear dice.
Here's a few unusual ones, including astrology dice, d1000, d100, d30, d24, pizza toppings, months, round dice, mood dice, weather dice, rock/paper/scissors dice, and a few odd finds.
I used the mood die once while DMing, when playing a mentally unstable character who had violent mood swings. Some day I hope to do something similar as a player.
This is a solution I had for playing in the car:
These bead boxes are made by Craft Mates; I bought mine at Joann. You shake the whole thing then look at the die you needed to see. The compartments lock shut so the doors shouldn't open while you're shaking it. Of course, 4e is difficult to play without minis, but if you run a low-combat module, you can probably get away with just writing on graph paper. I put them together when planning our last Florida trip, but we didn't get around to playing D&D, so I still don't know if it's a workable idea.