One thing I've periodically wondered is whether there are any kid's toy/action figures that are of the proper scale to be used as minis. Has anybody found anything that works?
I don't know if there is a Michael's craft store in your area, but they have various miniatures of animals, dragons, horses, hydras, giants, dinosaurs, etc. Most of the miniatures that would be used for player characters are bigger than the standard 1-inch square. Even so, I have used several of their products, and you might find something you like.
We've used lego minifigs as characters for a couple of years, and now and then as major NPCs. None of us can be bothered to collect or paint miniatures you see.
A lego minifig fits nicely enough on an inch grid wipe clean battlemap thingy.
I managed to source some nice round bases for them as well, which you can just see at the bottom of the above picture.
Here's a character portrait that the lego fig is an attempt to represent:
Many car models and toys have the scale listed, if you look for it, or search for it online. For car and motorcycle "miniatures" in my Shadowrun games, I use 1/43 scale items (e.g. City Cruiser cars). Any action figure in 1:43 or a similar scale would work, I imagine. I think military scale (1/33) is too large, and I don't know if there's a matching railroad model scale.
Back when I was a lad, you could get big bags of plastic soldiers, which were of roughly the right size. For games in a less modern setting, there were also bags of vikings, Native Americans, samurai, etc.
Whether these things still exist, I don't know, but they're in Toy Story, so I'd assume they do.
It's really a question of scale; it's theoretically possible to use ANY product line for minis as long as it's the only product line you're using, because they'll all be to scale with each other.
If you're doing D&D, though, then presumably you're operating in the traditional "1 inch equals 5 feet" scale, which mathematically works out to 1:60 scale, but in practice means that (as long as you're only talking about Medium-sized creatures) you're actually just looking for things which don't significantly occupy more horizontal space than a square inch. (This does, incidentally, include Lego minifigs; I've done games where the PCs were minifigs on little bases - the ones that came in the recent Toy Story Army Men set would have been perfect for this - and the monsters were D&D Minis.)
Incidentally, some online hobby retailers will allow you to search their product listings by scale., although that won't help you for things which are technically not-to-scale but which are close enough for minis representation.
We use everything. Kitchen magnets, Kinder Surprise toys, dice, glass pellets, 4-in-a-row chips (for effects)...
I've noticed that this works greas as long as you limit yourself to only using crap. We've used the few mini's we have managed to acquire over the years (heroquest, ebay, the gutter of the street, ...) in combination with junk and that kind of kills it for me sometimes. When you have an awesome model of a goblin, one of an orc and the orc mage is represented by a sugarcube... I would rather have them all be sugarcubes with their names painted on.
Anyway, I recommend 4-in-a-row chips or bottle caps for lasting effects (-2 on attack, +4 to all saves, ...). They stack well and you can distinguish by using red as bad and yellow as good.
Micromachines figures for Star Wars are close enough to 15mm to be used as such.
HO Scale is just a little under 15mm.
15mm isn't as popular as it used to be, but was once the norm for Sci-Fi minis games; it's still fairly popular in some circles for historicals, too. And if you're printing out your own maps, just halve the scale for use with 15mm or HO people.
On the other hand, you may want to look at some of the plastic minis by Zvezda and Ceasar in 20mm. And they're cheap... 20-30 figs for a $15 box... and durable, flexible plastic.
I used them for my L5R games.