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.