I have about 30 websites on Drupal. I'm looking for module or service where i can aggregate information from all websites in one place. I need information about number of nodes, comments, users, Drupal core and modules versions. Something like acquia insight but for more sites and cheaper :)
The Similar Module Review's Administration module wiki lists several options.
The following show some usage statistics, as well as Drupal core and module updates:
Take a look at Droptor. And the corresponding Droptor module.
30 websites looks like $696/year (the first one is free)
I have good experience with the Production monitor project.
The prod_check server module is enabled in the master site, while the prod_check client module is enabled in all sites. You may tracks modules, alerts, and even performance to verify that the tracked sites are in a good working state.
It's not as extensive as acquia insight, but the module has an API which allows you to extend it to provide more metrics.
Consider looking at the Forena module (disclosure: I'm a co-maintainer of it). It can be used to query the data in other databases. Various DBMS formats are supported, such as another Drupal database also (which your question appears to be about).
Here is how to get you going:
Forena is built of the idea of using SQL to get data out of a database and use XHTML and CSS to format it into web reports. For more details about Forena, 2 types of documentation are available:
Documentation that comes with Forena, which you can access right after install and enable of the module. Checkout the demo site for an online example of the current:
/reports/samples(these samples are fully functional, so make sure to experiment a bit with it, such as the drill downs available on the SVG Graph sample).