TinyCore (loads itself into RAM), Puppy, Proteus and Zolin Lite will all be happy with 512 MiB.
Edit: also antiX, QOS, Slax and DSL.
Archbang, Slitaz, Sparky and MX may not support x86, double-check.
Also gentoo has a full binary-only version now.
Edit2:
Interesting that Zorin says their regular flavors are now efficient enough that they'll run just fine on older hardware, so there's no need to continue Zorin Lite:
It’s now possible to run the non-Lite editions of Zorin OS on computers with as little as 2 GB of RAM and on machines as old as 15 years, with higher performance than the Lite edition in some tasks.
Emphasis mine. Wow, only 2 GiB...