I've recently been looking at alternatives to F5 and I remember from this year's QCon a lot of people mentioning Zeus so I thought I'd give them a look.
So far they look promising; being able to run Zeus on your own generic hardware could be a cost saving in the long run. For example we had to buy some spare PSUs out of fear we wouldn't be able to buy them in the future.
One feature I really like the sound of is the "webcache!refresh_time". It smooths out the load to your backend by only sending one request to your appserver while serving the rest from cache. For sites that must have low cache times this makes a lot of sense. If you get 30 requests per second for an item, you only send one request to your backend as it expires from cache.
Turns out F5 also have a similar sounding feature called Fast Cache but for us it would be an additional module (read additional cost).
If anyone has experience with either I'd love to hear their thoughts.