It's one of besk keept "secrets" of Joomla 2.5+ - there is a built in password strenght meter, ready to be used. And some are selling for good money - and others offering free plugins - to let you unleash the hidden power.
But if you aren't afraid to make your hands dirty with some PHP code, here is how you can do this:
1. Do a backup.
2. Check it, and do it again (yeah, we will do something qualifying a core hack - you can't be too cautious)
3. Grab your FTP client and favourite editor, hunt down this file:
/components/com_users/models/forms/registration.xml
4. Find this section:
<field name="password1" type="password"
autocomplete="off"
class="validate-password"
description="COM_USERS_DESIRED_PASSWORD"
field="password2"
filter="raw"
label="COM_USERS_REGISTER_PASSWORD1_LABEL"
message="COM_USERS_REGISTER_PASSWORD1_MESSAGE"
required="true"
size="30"
validate="equals"
/>
5. Add this line:
strengthmeter="true"
For example like this:
<field name="password1" type="password"
autocomplete="off"
class="validate-password"
description="COM_USERS_DESIRED_PASSWORD"
field="password2"
filter="raw"
label="COM_USERS_REGISTER_PASSWORD1_LABEL"
message="COM_USERS_REGISTER_PASSWORD1_MESSAGE"
required="true"
size="30"
validate="equals"
strengthmeter="true"
/>
6. Save.
7. Enjoy!