Hi Moran,
I like olark a bunch. The good thing is that you can use it for free for up to 20 conversations. It is plenty to figure out whether you like it or not.
I don't really love Sigma Live because out of the box it looks a bit outdated. But if you know a bit of HTML you can pretty it up.
Here is a video about Sigma Live that I have created before. There are a few step to get it working.
Cheers,
Aderson