VBulletin free/affordable alternative?

For a budget which is limited at startup, so is not allowing for Vbulletin at his launch stage (hopefully will be later on) can anyone suggest a messageboard which is either is free, or very affordable but which has good features and is comparable?

if applicable, please also mention if you know anything about how its security, or if you yourself have used it in the past and what your pros/cons are.

Thanks in advance for any help you can give.

The good thing is vbulletin makes it easy for you to upgrade later. Here are some (mostly free)
phpBB would be at the top of most lists for free forum software.