I have been recently exposed to UltraMonkey as part of my daily job on SpringRing (now Aloha). UltraMonkey is a solution for load balancing and high availability of network services. We are using it to load balance the SimpleSipStack instances to experiment the stack scalability. It works quite well and it has - good! - native SIP support.
Well, UM is essentially a bunch of Linux tools that you configure to implement the topology you want (Load Balancer, Load Balancer + High Availability, ...).
Downloads are available tarballs or RedHat rpms or Debian packages, so installation in our Suse 9 was quite far from straightforward but in the end configuration is very easy, at least for the load balancer topology.
Pros: it's easy and quite powerful, it runs on Linux, it's cheap
Cons: you need to be more than a newbie on networking to really understand how it works and how to best make use of it. On the good side, it's a good opportunity to learn new stuff