Tools to monitor Oracle Database Performance on Standard Edition
Note: I've never used this, so please edit this answer if you have.

Lab128






  Lab128 provides additional savings by implementing ASH functions and
  not requiring the purchase of ODP.


Pros:


Response Time Analysis
RAC support
Agentless
Only $299 for a named user connecting to two instances and only $499 for a named user connecting to an unlimited number of instances.


Cons:


Interface isn't very polished.
Lack of alerts.
One writer/maintainer.
No web interface.
Not server based.