For those of you who've been logging in to the Minecraft server, you may have noticed that on occasion (sometimes more frequently than occasionally), your client refuses to connect with an "invalid server key." As it turns out, this is a bug in the client-side code that Mojang has yet to fix. Never fear, for sk89q of Bukkit fame (he wrote WorldEdit, WorldGuard, and tons of others) has a fix.
For those of you who aren't familiar with Java or any of its intricacies and don't wish to muck about with your minecraft.jar, there is a simpler way of installing this fix in the form of an installer you can download (Windows only).
Thus far, several of us have tried it and it works well. If you're experiencing this issue, try the installer first. Linux and Mac users will need to unzip the jar file, remove the signatures from META-INF (everything but MANIFEST.MF), and replace the abp.class in the .jar file with the one here. You'll then need to re-zip the .jar if you extracted it, and rename the new minecraft.zip to minecraft.jar.