Recently I've taken to using the eight-pointed star as an explicit symbol of Chaos in my games. It fits very well with the idea that Chaos is an explicit "side" that you can be explicitly for or against, in a broad cosmic war of which factions in the campaign world are just general reflections. Chaos, as I use it, is a powerful force led by many demonic and strange Chaos lords, who are as often as not at cross purposes with one another, as symbolized by the eight directions of the Chaos arrows. It's good and Moorcockian and flavorful. I've had NPCs use it for decoration, including unholy symbols and tattoos.
This Chaos symbol comes from the influence of Swords & Wizardry, even though ultimately it's Moorcock's. S&W also has a circle for Law, which is different from Moorcock who used a single straight arrow. I like the circle, though, because it's an easy graffito. A Law circle can be drawn around the Chaos arrows, like the three downward-pointing arrows of the Eiserne Front were designed to counter Nazi swastikas.
I don't have a symbol for Neutrality that I like. Two wide horizontal bars might work for people who are "actively neutral" (that is, opposed to both Law and Chaos). The thing is, Neutrality in many ways is sort of a catch-all of unaligned, selfish, people aligned to the Earth, people who want a balance, and animals that don't have the intelligence to have an alignment.
But using the circle and the eight-pointed star has had me thinking about other symbols with clear resonances. The Elder Sign, whether it is the pentagram of Derleth or Lovecraft's "tree branch" that has five tines pointing off of a central line, is a classic symbol to ward off things that are not from this world. The Yellow Sign is a classic sigil that is immediately ominous. And I think players might revolt if they saw the Duvan'ku Dead Sign from Death Frost Doom. Or at least, if they know what it means.
The World of Greyhawk has some good entrance runes, if a bit unique to its setting. But on a similar vein I think it would be really fun to start using the classic hobo signs around a megadungeon or wilderness sandbox type environment; obviously the railroad-related ones are no good but most of them would be an interesting way to make the PCs seem more like they're part of a larger world where they aren't the only adventurers about. Simple trail signs can also serve a similar purpose, though they're more likely in the wilderness.
Signs and symbols are also a great way to introduce a mystery into the campaign. Just throw an unknown sigil at the characters, and they will naturally investigate it – whether through a sage, or asking around, or what have you. These kinds of things are a good way to pique the curiosity of players.
I'd be curious about signs and symbols people have used in their own campaigns, and what the impact has been.

