Good one-way sync program for Windows?

I would highly advise you use Microsoft SyncToy, you can setup one way partnerships and it can do exactly what you want!

Robocopy is good -

e.g. to synchronize folders,

robocopy <source> <dest> /MIR

to see other options run robocopy /?

There's also a UI wrapper called RichCopy - doesn't seem to have much of a home, closest thing. More Info. Blog updates.

As Wil mentioned, SyncToy is a great GUI tool for this. If you'd rather a command line program which you could use in batch files and schedules tasks, take a look at robocopy. It is a command line robust copying tool and is built into Windows 7. It will only copy files which have changed, works over the network or locally, has wildcard support, can exclude specific filetypes, individual files, or directories, and can run in restartable mode for if it gets interrupted. A very powerful tool, and not too difficult to use. Run robocopy /? in the command line for documentation.