If you need to reset the password on Mac OS X Lion but you don't have any secondary admin accounts, it's actually pretty easy. Even if you don't have an install disc.
Different versions of OS X require different stuff but this post specifically applies to Lion. (That's the only OS I tested it on, at least.) So here are the instructions:
1. Power up the Mac
2. As soon as you see the gray screen, hold down Command-S
3. Let go once you see text on a black background
4. Once the text stops scrolling, hit enter
5. Enter this command: mount -uw /
6. Enter this command: rm /var/db/.AppleSetupDone
7. Enter this command: reboot
8. Go through the setup wizard and choose defaults. Do not import any data, etc. Name your user something like temp
9. Login as the new user
10. Navigate to System Preferences>Users & Groups, click the padlock icon and authenticate
11. Select the user who you want to reset the password for and click Reset Password
12. After resetting the password, logout
13. Login as the user that you reset the password for
14. When you get the keychain dialog, you will likely just want to just choose new, but it's up to you
15. Delete the temp user you created
If you are not sure what version of OS X you have, you may want to type the following command right after step 4: uname -a The information displayed can be checked against here or by googling to determine the operating system.
Although it is always a good idea to have backups and I can't be held liable for data loss, etc., the method above is one that should not harm any data at all. So don't freak out. This method should not remove data from the user account you are trying to get into.