Firefox: If you've been known to fat-finger URLs and wind up going to lifehaker instead of lifehacker, NBar is a Firefox extension that uses string matching and logic to figure out where you meant to go, correct the URL, and send you on your way.
The add-on is a quick install, and once it's running, it'll automatically correct not just common URL errors, but some more complicated ones as well. It can recognize when you've inadvertently swapped keys, like typing "kc" instead of "ck," or have just misspelled a word or URL entirely. Granted, it doesn't take much to fix a typo, but having a tool do it for you is a huge help. It's worth noting that the add-on won't correct the URL if you go ahead and press enter—it just suggests the right URL in the drop-down as you're typing the wrong one. It's still up to you to select the one you actually meant to visit.
NBar isn't the only add-on that does this (previously mentioned URL Fixer is similar, but less powerful), although it's one of the best. Chrome and Firefox users can use AdBlock Plus' built-in typo protection, although it's geared towards helping you avoid malware-laden domains that are a letter off of popular ones. Alternatively, you can switch over to OpenDNS on your home PC or router, which will automatically redirect mistyped URLs to the right ones for you.
nbar (Free) | Mozilla Add-Ons via Ghacks