If you have Opera check if User JavaScript is enabled, see Help > About Opera or opera:about. If User JavaScript is enabled, the location of the User JavaScript files will be shown as the "User JavaScript files"; listed under "Paths". To enable user JavaScript, use Tools > Preferences > Advanced > Content > JavaScript options, and select the directory where you will put your User JavaScript files. [read more]

Windows only: The latest version of Google Chrome adds support for user scripts, but if you want to enable them you'll need to follow a couple of quick steps:
Step 1: You'll want to open up your favorite file manager and browse down to the following path for your Windows version:
Windows XP: C:\Documents and Settings\YourUserName\Local Settings\Application Data\Google\Chrome\User Data\Default\
Windows Vista or Windows 7: C:\Users\YourUserName\AppData\Local\Google\Chrome\User Data\Default\
Replace C: with the drive windows is installed in and YourUserName with the windows users you installed chrome with.
Step 2: If there isn't a User Scripts folder inside the default folder then create one, this is the folder where User Scripts will be stored.
Step 3: The last step is to modify the Google Chrome application shortcut to include the --enable-user-scripts after the executable path.
With these steps you will be able to run user scripts in Google Chrome. [read more]