The key to getting your software out there is to make sure it is well written, useful and attractive. Then....
- Submit it to versiontracker
- Submit it to Apple (Mac OS X Downloads)
- E-mail magazines (Macworld etc.) and see if they'd like it on their CDs
- Press-release announcing it to macnn, macminute etc.
- Advertising if you have the budget (people don't like popups though!)
If you have an good application word of mouth will also help it spread.