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.