The Logfile Navigator, lnav for short, is a curses-based tool for viewing and analyzing log files. The value added by lnav over text viewers or editors is that it takes advantage of any semantic information that can be gleaned from the log file, such as timestamps and log levels. Using this extra semantic information, lnav can do things like interleaving messages from different files, generate histograms of messages over time, and provide hotkeys for navigating through the file. These features are meant to allow the user to quickly and efficiently focus on problems.
libKISSlog is a trivial lightweight C++ template library designed and written according to the KISS (Keep It Simple and Straightforward) principle. It leans heavily on STL for keeping its implementation as simple as its usage, and tries to provide C++ developers with a lightweight, paradigm-pure, and flexible alternative to logging libraries which use design and/or implementation decisions which at least the author of libKISSlog believes to be questionable. Its easiest to explain why libKISSlog would be suitable for your needs by listing the things which libKISSlog does not choose to use or do: no singletons or other forms of mutable global state, no macros, no attempt to fit the Java runtime everything model onto a C++ library, no attempt to be a Java-style (bloated) framework, no attempt to make the choice for you of whether you need thread safety, and no compromise on simplicity in order to facilitate questionable inner-loop logging practices.
Pantheios is a C/C++ logging API library. It offers an optimal combination of complete type-safety, very high efficiency, genericity, and extensibility. It is simple to use and extend, highly portable (platform and compiler independent), and it upholds the C tradition of only paying for what you use. Pantheios supports logging of message statements of arbitrary complexity, consisting of heterogenous types. Pantheios supports filtering of log messages based on severity level.
pdr (personal data recorder) and pdx (personal data expert) are applications that collect and evaluate mostly numeric personal data. The intended use case is for logging individual medical data (blood sugar, blood pressure, body temperature, weight, heart rate, and medications), but they can also be used for technical, sports, fitness, weather, environmental, or financial data. They work on a continuous flow of numbers in time that you want to monitor. Every data item can also be commented by text. pdr can use Twitter and email accounts for data input, which makes mobile phones usable for data input. It is one of the few free applications for diabetics.
Libptytty is a small library that offers pseudo-TTY management in an OS-independent way. It was created out of frustration over the many differences of PTY/TTY handling in different operating systems for use inside "rxvt-unicode". It also offers session database support (utmp and optional wtmp/lastlog updates for login shells) and supports forking a proxy process after startup and dropping privileges in the calling process. It offers C++ and C-only APIs.
NoBug is a library which provides assertions, logging statements, and annotations for C and C++ programs. It includes pre-/post-condition and invariant checks, as well as generic assertions. Checks are enabled based on build-levels and scope tags. It also includes debugger support (valgrind), dumping of data structures, logging your application's activities, runtime customizable logging via environment variables, different logging targets (ringbuffer, stderr, syslog, debugger, etc.), and annotation of your source code regarding known bugs, things to do, and planned things. It can track resources and detect deadlocks.