Recommended Log analysis tool

Hi,

Do you know any log analysis tool either commercial or opensource or any place 
or thread/links where this topic is being discussed? There's only one 
requirement, it must run on *nix platform. Right now we are looking at 
sawmill and manageengine. I have until the end of the month to decide which 
one to proposed or if you know anything that, based on experience, can 
do "better" or any guidlines as to how do I go about choosing my log analysis 
tools....
Right now, we have a syslog-ng which collects logs from cisco swtiches, 
routers and junipers... Now the manageengine has its own syslog daemon and it 
can also analyze logs from raw files just like what sawmill does...
Is there any rule here when processing log files in "almost" realtime vs 
processing log files which was collected by other means?

Thanks a lot.
jay