There are several parameters that can be passed in to the SRS Report viewer through query string parameters. Some of them are listed below:

Change the way it is rendered:
rs:Command=Render& (4 types of re:Commands)

Pass in parameter values:
PARAMNAME1:isnull=true&PARAMNAME2=PARAMVALUE2&

Control the type of output:
rs:Format=PDF

Control what toolbar(s) show in the viewer:
rc:Toolbar=False
rc:Parameters=False