The AdminExile Plugin has long been a favored and highly rated extension in the JED. Read the reviews, check out the 9 pages of documentation, and then try it yourself.

Key features:
- Access key(s) - key only, or key + key value. Others provide one or the other. AdminExile provides BOTH.
- Front-end Restriction - Restrict certain accounts from logging into the front-end with accounts intended only for back-end use.
- Lost Key Recovery - Useful for individuals using extremely difficult keys, or teams who change the keys frequently.
- Stealth Mode - Prevents tell-tale signs that something exists at /administrator, like the session cookie!

There are far too many features to describe in this page. Visit the documentation link to get the bigger picture.

The JED won't let me post a demo link, because the extension forces a 404 page response because you don't have the keys. Just go to my website and try to access /administrator - don't worry, I turned brute-force detection off.