diff --git a/eventlet/hubs/hub.py b/eventlet/hubs/hub.py
index 9ae5165..ca52c48 100644
--- a/eventlet/hubs/hub.py
+++ b/eventlet/hubs/hub.py
@@ -22,6 +22,7 @@ THE SOFTWARE.
 """
 
 import bisect
+import weakref
 import sys
 import socket
 import errno
@@ -237,14 +238,20 @@ class BaseHub(object):
     def track_timer(self, timer):
         current_greenlet = greenlet.getcurrent()
         timer.greenlet = current_greenlet
-        if current_greenlet not in self.timers_by_greenlet:
-            self.timers_by_greenlet[current_greenlet] = {}
-        self.timers_by_greenlet[current_greenlet][timer] = True
+        self.timers_by_greenlet.setdefault(
+            current_greenlet,
+            weakref.WeakKeyDictionary())[timer] = True
+
+    def timer_finished(self, timer):
+        try:
+            del self.timers_by_greenlet[timer.greenlet][timer]
+            if not self.timers_by_greenlet[timer.greenlet]:
+                del self.timers_by_greenlet[timer.greenlet]
+        except KeyError:
+            pass
 
     def timer_canceled(self, timer):
-        del self.timers_by_greenlet[timer.greenlet][timer]
-        if not self.timers_by_greenlet[timer.greenlet]:
-            del self.timers_by_greenlet[timer.greenlet]
+        self.timer_finished(timer)
 
     def prepare_timers(self):
         ins = bisect.insort_right
@@ -279,21 +286,26 @@ class BaseHub(object):
                 except:
                     self.squelch_timer_exception(timer, sys.exc_info())
             finally:
-                try:
-                    del self.timers_by_greenlet[timer.greenlet][timer]
-                except KeyError:
-                    pass
+                self.timer_finished(timer)
         del t[:last]
 
     def cancel_timers(self, greenlet, quiet=False):
         if greenlet not in self.timers_by_greenlet:
             return
-        for timer in self.timers_by_greenlet[greenlet]:
+        for timer in self.timers_by_greenlet[greenlet].keys():
             if not timer.cancelled and not timer.called and timer.seconds:
                 ## If timer.seconds is 0, this isn't a timer, it's
                 ## actually eventlet's silly way of specifying whether
                 ## a coroutine is "ready to run" or not.
-                timer.cancel()
+                try:
+                    # this might be None due to weirdness with weakrefs
+                    timer.cancel()
+                except TypeError:
+                    pass
                 if _g_debug and not quiet:
                     print 'Hub cancelling left-over timer %s' % timer
-        del self.timers_by_greenlet[greenlet]
+        try:
+            del self.timers_by_greenlet[greenlet]
+        except KeyError:
+            pass
+
diff --git a/eventlet/hubs/libev.py b/eventlet/hubs/libev.py
index 6e5d421..9899336 100644
--- a/eventlet/hubs/libev.py
+++ b/eventlet/hubs/libev.py
@@ -115,11 +115,22 @@ class Hub(hub.BaseHub):
         eventtimer.start()
         self.track_timer(timer)
 
+    def timer_finished(self, timer):
+        try:
+            timer.impltimer.stop()
+            del timer.impltimer
+        # XXX might this raise other errors?
+        except (AttributeError, TypeError):
+            pass
+        finally:
+            super(Hub, self).timer_finished(timer)
+
     def timer_canceled(self, timer):
         """ Cancels the underlying libevent timer. """
         try:
             timer.impltimer.stop()
-        except AttributeError:
+            del timer.impltimer
+        except (AttributeError, TypeError):
             pass
         finally:
             super(Hub, self).timer_canceled(timer)
diff --git a/eventlet/hubs/libevent.py b/eventlet/hubs/libevent.py
index 7492158..bb5ab6a 100644
--- a/eventlet/hubs/libevent.py
+++ b/eventlet/hubs/libevent.py
@@ -124,11 +124,22 @@ class Hub(hub.BaseHub):
         eventtimer.add()
         self.track_timer(timer)
 
+    def timer_finished(self, timer):
+        try:
+            timer.impltimer.delete()
+            del timer.impltimer
+        # XXX might this raise other exceptions? double delete?
+        except (AttributeError, TypeError):
+            pass
+        finally:
+            super(Hub, self).timer_finished(timer)
+
     def timer_canceled(self, timer):
         """ Cancels the underlying libevent timer. """
         try:
             timer.impltimer.delete()
-        except AttributeError:
+            del timer.impltimer
+        except (AttributeError, TypeError):
             pass
         finally:
             super(Hub, self).timer_canceled(timer)
diff --git a/eventlet/timer.py b/eventlet/timer.py
index 1018320..abeb868 100644
--- a/eventlet/timer.py
+++ b/eventlet/timer.py
@@ -29,7 +29,7 @@ useful for debugging leaking timers, to find out where the timer was set up. """
 _g_debug = False
 
 class Timer(object):
-    __slots__ = ['seconds', 'tpl', 'called', 'cancelled', 'scheduled_time', 'greenlet', 'traceback', 'impltimer']
+    #__slots__ = ['seconds', 'tpl', 'called', 'cancelled', 'scheduled_time', 'greenlet', 'traceback', 'impltimer']
     def __init__(self, seconds, cb, *args, **kw):
         """Create a timer.
             seconds: The minimum number of seconds to wait before calling
@@ -40,7 +40,6 @@ class Timer(object):
         This timer will not be run unless it is scheduled in a runloop by
         calling timer.schedule() or runloop.add_timer(timer).
         """
-        self.impltimer = None
         self.cancelled = False
         self.seconds = seconds
         self.tpl = cb, args, kw
@@ -57,28 +56,28 @@ class Timer(object):
             secs, cb, args, kw)
         if _g_debug and hasattr(self, 'traceback'):
             retval += '\n' + self.traceback.getvalue()
-        return retval            
+        return retval
 
     def copy(self):
         cb, args, kw = self.tpl
         return self.__class__(self.seconds, cb, *args, **kw)
-    
+
     def schedule(self):
         """Schedule this timer to run in the current runloop.
         """
         self.called = False
         self.scheduled_time = get_hub().add_timer(self)
         return self
-        
+
     def __call__(self, *args):
         if not self.called:
             self.called = True
-            if self.impltimer is not None:
-                del get_hub().timers_by_greenlet[self.greenlet][self]
-
             cb, args, kw = self.tpl
-            cb(*args, **kw)
-        
+            try:
+                cb(*args, **kw)
+            finally:
+                get_hub().timer_finished(self)
+
     def cancel(self):
         """Prevent this timer from being called. If the timer has already
         been called, has no effect.