dstat is a versatile replacement for vmstat, iostat, netstat, nfsstat, and ifstat. It includes various counters (in separate plugins) and allows you to select and view all of your system resources instantly; you can, for example, compare disk usage in combination with interrupts from your IDE controller, or compare the network bandwidth numbers directly with the disk throughput (in the same interval).
RRDutil is a a tool to collect statistics (typically every 5 minutes) from multiple servers, store the values in RRD databases (using RRDtool), and plot out pretty graphs to a Web server on demand. The graph types shown include CPU, memory, disk (space and I/O), Apache, MySQL queries and query types, email, Web hits, and more.
Spike Asset Manager (SAM) is a cross-platform framework for probing a system for components and reporting them. It includes a driver file that probes for components commonly found in a LAMPJ stack (Apache, MySQL, PHP, Tomcat, etc). It can find multiple versions that are installed, query the RPM database, and indicate whether components are running.
Aranha is an application that uses an XML-RPC layer to retrieve information from one or more servers. The type of information gathered is implemented via plugins, and can be displayed or archived in several different ways. The data is gathered in Round Robin Databases (integrated with rrdtool) and can be accessed using a XUL interface available from the system. Plugins are very easy to write.
md5i creates an MD5 database of all files in a directory structure and allows you to easily recheck the contents and report if any files have changed. It does not detect removed files, and only computes an MD5 sum if the "last modified" timestamp of a file has changed since the last execution by default. Wrapper scripts for Unix and Windows are included.