If you're too lazy to actually come up with unique passwords for each site and you happen to have OpenSSL installed (who doesn't?), you can automatically figure out all your passwords only having to remember one.
Come up with a base password, for the sake of argument let's say ABCDEF. For each site, append the name of the site to your base password. E.g., for Slashdot, it's ABCDEFslashdot. "echo ABCDEFslashdot | openssl sha1" yields your password of 040b6c2fb4d5858ad21810deb8e9ee2eb804e2a7. From that password it is intractable to determine what your base password was and hence what your other passwords are.
Some sites require special characters or, even worse, have maximum password lengths (which would suggest they're storing your password in plaintext, yikes). Fuck those sites.