I highly recommend CFEngine Enterprise, which is an open-core product (open source community edition, closed source enterprise edition) which is free for up to 25 hosts connected to a single "hub."
CFEngine is built on a "pull" model. CFEngine 3 has been around since 2008, and has zero vulnerabilities listed in the National Vulnerabilities Database. (There are a very few vulnerabilities listed for CFEngine 1 and 2, but none for CFEngine 3 which was fully rewritten and greatly improved.)
While you can use CFEngine for configuration (its very name comes from "configuration engine" and it is the original configuration management software), it is still extremely useful even when used only for inventory.
Out-of-the-box details that are inventoried include:
- Kernel version
- OS flavor (e.g. RedHat 6.5)
- CPU information
- Disk utilization
- Listening TCP ports
- All software packages installed and all software updates available, with package versions and architectures
- (Out of the box this works for
yum based systems; if you use something else you can use the package module API to write a CFEngine interface for another package management system in your language of choice.)
Plus, of course, you can write your own CFEngine policy to inventory whatever else you need to inventory.
CFEngine Core (the open source community product) handles server configuration, but doesn't have inventory built in (although you can hack something together if you really want to go to the trouble).
CFEngine Enterprise has a Postgres database built in (on the hub only, of course, not on the agents) which contains inventory and configuration compliance information for all connected hosts, and makes it available through a fairly full-featured web UI.
Note: I am not affiliated with the CFEngine company. I do deliver training in the use of CFEngine and have contributed to the open source CFEngine Core. I am biased in favor of CFEngine over other products (e.g. Puppet), but only because I honestly think it is a better product—not for financial reasons.