I'm using git with trac and I need some simple code review tool. I just need the ability to write some comments on some lines on each commit.
I've googled a litte and found gerrit, but It seems too complex for my needs. I also found trac plugin http://trac-hacks.org/wiki/CodeReviewPlugin, I will try it. But maybe you can pont to some better solution.
You've reached the solutions being commonly used. But you should reconsider using Gerrit. It's been forked from a Rietveld which was inspired from Mondrian that Google uses internally. But if you still think you don't need it then the trac plugin you've pointed to should be enough.
For other solutions you may take a look at http://malevich.codeplex.com/
The new tools under discussion these days is Review-board. You can read about it's review here
I work at a company which has come up with an "alternative" to gerrit. We use codestriker, for which I had to write some basic git support. It may provide what you're asking for; it can suck in the changes from one commit, and let you write some comments on the diff.
That said, we've ended up in this situation largely because the integrators and people in charge don't actually know how to use git. Our workflow is so incredibly broken that it would be quite difficult for us to use gerrit. I would much rather be using gerrit than codestriker, and I think you should probably consider doing the same.