What steps will reproduce the problem?

1. Start the server.

What version of the product are you using?
SpamHammer 1.7b

Do you have an error log of what happened?

Could not load 'plugins\SpamHammer.jar' in folder 'plugins'
org.bukkit.plugin.InvalidPluginException: java.lang.NoClassDefFoundError: org/bukkit/event/player/PlayerListener
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:148)
at org.bukkit.plugin.SimplePluginManager.loadPlugin(SimplePluginManager.java:287)
at org.bukkit.plugin.SimplePluginManager.loadPlugins(SimplePluginManager.java:211)
at org.bukkit.craftbukkit.CraftServer.loadPlugins(CraftServer.java:206)
at org.bukkit.craftbukkit.CraftServer.(CraftServer.java:182)
at net.minecraft.server.ServerConfigurationManager.(ServerConfigurationManager.java:53)
at net.minecraft.server.MinecraftServer.init(MinecraftServer.java:156)
at net.minecraft.server.MinecraftServer.run(MinecraftServer.java:420)
at net.minecraft.server.ThreadServerApplication.run(SourceFile:490)
Caused by: java.lang.NoClassDefFoundError: org/bukkit/event/player/PlayerListener
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClassCond(Unknown Source)
at java.lang.ClassLoader.defineClass(Unknown Source)
at java.security.SecureClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.defineClass(Unknown Source)
at java.net.URLClassLoader.access$000(Unknown Source)
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:41)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
at com.dumptruckman.spamhammer.SpamHammer.(SpamHammer.java:26)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at org.bukkit.plugin.java.JavaPluginLoader.loadPlugin(JavaPluginLoader.java:144)
... 8 more
Caused by: java.lang.ClassNotFoundException: org.bukkit.event.player.PlayerListener
at java.net.URLClassLoader$1.run(Unknown Source)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(Unknown Source)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:41)
at org.bukkit.plugin.java.PluginClassLoader.findClass(PluginClassLoader.java:29)
at java.lang.ClassLoader.loadClass(Unknown Source)
at java.lang.ClassLoader.loadClass(Unknown Source)
... 27 more

Please provide any additional information below.

I tested SpamHammer 1.7b with Craftbukkit R6 (MC: 1.2.2 ) and it will break.

I know that your plugin is from 1.7.3 but it did work for 1.0.1 and 1.1.