One of the differences between Tiger and Leopard is that the former had an Options action in the login pane that allowed users to choose to connect to AFP shares with clear text (unencrypted) passwords. This option is missing from the Leopard AFP connection pane.
Unfortunately, many Appleshare servers only support clear text passwords -- for instance, the Qnap TS-109 NAS, which has an implementation of netatalk built in to it. Luckily, until the server supports connections with encrypted passwords, it is possible to configure Leopard to support clear text passwords.
To do so, disconnect from any AFP shares you are using, then navigate to ~/Library » Preferences » com.apple.AppleShareClient.plist. Double-click on the file to open the Property List Editor, which is part of the Developer Tools (the editor from the 10.4 tools works fine). Then just change the values of the keys afp_cleartext_allow and afp_cleartext_warn to Yes from their default value of No. Close the file, restart, and the next time you connect to an Appleshare server that only supports unencrypted passwords, you'll be able to do so.
It is also possible to edit these keys using the defaults system. The commands to do that, in Terminal, are:
$ defaults write com.apple.AppleShareClient "afp_cleartext_allow" -bool YES
$ defaults write com.apple.AppleShareClient "afp_cleartext_warn" -bool YES
[robg adds: I haven't tested this one.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20071028025409750