These are people I've encountered on the web, often several times for unrelated things.

Ross Comer

Went to high school with him. He wrote this program, ABSTAR (absent / tardy), and sold it to all the Ohio public schools. It did auto-completion of student's names as you typed them in. He went on to work on Excel, and is still at Microsoft.

Paul Crowley

I've only encountered Paul doing cryptography. But he takes it seriously and practically, perhaps because he's actually able to put it to use. We worked on RC4 when it first went public. I've seen him commenting on the recent European stream ciphers too.

David Eppstein

I ran into him due to pentagonal tiles, and again for other math in his geometry junkyard, then again for his photo gallery.

John Kelsey

Works for Bruce Schneier. We started on cryptography about the same time. He went on to be a professional, while I dropped out of the scene. He's worked on Twofish and HELIX.

Don Knuth

Don wrote "The Art of Computer Programming". He's done tons of stuff on hashing and pseudorandom numbers, including popularizing x[i]=x[(i+24)%55]+x[(i+31)%55]. He wrote the TeX formatting language, which is why research papers have looked the way they do for several decades. He gives interesting talks occasionally at Stanford. And he was on my dad's undergraduate dormitory floor at Case Institute of Technology (now Case Western Reserve University). He abandoned email in the early 1990's; wise man.

Randall Parker (Future Pundit)

Oil, genetics, autism, aging, he's asking the same questions I'm asking, and usually getting the same answers.

Ed Pegg (mathpuzzle.com)

I invented a bunch of pentagonal tiles, but Ed invented at least one of them before I invented it. (I don't think any of the tiles I invented I invented first.) I've run into him for gravitation, and pointers to the math olympiad, and any other of mathematical things. I bought a tiling puzzle he had manufactured that contained that tile I reinvented.

Colin Plumb

Hacker extraordinaire. I ran into him doing both random numbers and hashing. He proposed the requirements for my one-at-a-time hash. I haven't spotted anything from him in years.

John Savard

Pentagonal tiles, cryptography, combinatorics, genetics, gravitation ... I see him on usenet all the time.

Bruce Schneier

The public face of cryptography.

David Wagner

Another cryptographer often spotted on the web. High accuracy rate.

John Walker (Fourmilab)

This guy founded autodesk. He also maintains Fourmilab, which has some gravity simulators.