hledger is a suite of applications, tools and libraries.
The main hledger code repository is github.com/simonmichael/hledger
There are also various hledger addons maintained as separate projects with their own repos.
Within the main repo, there are a number of separate cabal packages, making it easier to pick and choose parts of hledger to install or to package. They are:
Core data models, parsing, standard reports, and utilities. Most data types are defined in Hledger.Data.Types, while functions that operate on them are defined in Hledger.Data.TYPENAME. Under Hledger.Read are parsers for the supported input formats. Data files are parsed into a Journal, which contains a list of Transactions, each containing multiple Postings of some MixedAmount (multiple single-CommoditySymbol Amounts) to some AccountName. When needed, the Journal is further processed to derive a Ledger, which contains summed Accounts. In Hledger.Reports there are standard reports, which extract useful data from the Journal or Ledger.
Here's a diagram of the main data model:
Dev sprint/party 2015/10/10: discussion