Removing personal data from OSX without deleting user or reinstalling OS
I would remove all files from the the obvious: 


Desktop  
Downloads
Movies
Music
Pictures
Public
Sites


Then inside your Library Folder you could send  everything inside the following folders to the trash: 


Application Support (this will remove all settings and data from any application you used) 
Autosave Information
Caches
Calendars
Cookies
Favourites
Fonts
Google
Images (iChat stores images here)
Keychains (all your passwords that are encrypted into your keychain)
Logs
Mail (all your email)
Mail Downloads (all your attachments from mail)
Preferences 
Preference Panes
PubSub
Safari


Once you've sent all relevant files to the trash. Secure Empty Trash from the Finder menu.  

Finally, you can over-write any left-over file data on the hard drive by doing the following:


Create a large file that takes up all remaining disk space. Let's say that it's called '~/bigfile'. You can do the following using the Terminal.app or X11.app to consume all remaining free space on your hard drive: 
yes "Foo Bar Baz" &gt; ~/bigfile
Secure-erase the file:
srm -m ~/bigfile


This in effect simulates the Disk Utility's "Erase Free Space" feature without needing admin access. More on this method can be found here.

Finally, you might want to cover your tracks a bit by deleting your shell history. Assuming that you are using the default Bash shell, you can delete your ~/.bash_history file.