Greg Egan's website has a little Java applet to visualise what happens to light around a black hole, dated 2001.

He's got a bunch of other fun stuff there, explaining/"demonstrating" the strange physics (real and theoretical) used in his books and stories.

It'll be deprecated in NetBSD 5.0 (which will be out Real Soon Now; it's well into the release candidate stage), and removed in 6.0; it's already been removed from -current (the CVS HEAD). 5.0 is also the first release to have WAPBL, the new journaling scheme.

zsh tab-completion. $[2**30/1e9] TAB, for example.

(Yes, really.)

The idea is that programs are written in a heavily restricted subset of x86 that can be proven safe, where "safe" here basically boils down to not being able to make system calls or otherwise interact with the world without going through the sandbox library. The program might still be compromised by a third party if it's badly written, but then the attacker won't be able to escape the sandbox either.

Smalltalk did this in the '70s, and the idea goes back to APL in the early '60s.

And, perhaps more to the point, Multics, also in the '60s.