which ones?
Some, if you're talking about stuff that loads with your user account, can be turned off in System Preferences > Accounts > Login Items
Some start up as a result of other Preference choices - If you have Personal Web Sharing enabled in the Sharing pref for example, then it will boot Apache at startup
Some other apps that need to boot below user level will add themselves to /Library/StartupItems
Most boot level startups are included in /System/Library/StartupItems, but it's best not to fool with these directly and to find what Prefrence or App has put them there and remove them from there