OpenBeOS was the initial name for the open-source project that sought to reverse-engineer BeOS. The name was changed to Haiku to avoid any legal trouble that might come from using the BeOS name. See HaikuOS.

System API

OpenBeOS KernelKit
OpenBeOS DeviceKit
  OpenBeOS.UsbKit
POSIX
STL - C++ Standard Template Libary

Application API

OpenBeOS ApplicationKit
OpenBeOS InterfaceKit
OpenBeOS PrintingKit
OpenBeOS SupportKit
OpenBeOS TranslationKit

File System API

OpenBeOS Storage Kit
BeOSFileSystemAPI
FileSystem - BeFS

Multimedia and MIDI API

OpenBeOS.MediaKit
  OpenBeOS.MediaKit.Activities
   OpenBeOS.MediaKit.Activities.PlayVideoFileFromHardDisk
   OpenBeOS.MediaKit.Activities.RecordOrPlayFromSoundCard
   OpenBeOS.MediaKit.Activities.StreamingStress
  OpenBeOS.MediaKit.APIChanges
  OpenBeOS.MediaKit.Controllable
  OpenBeOS.MediaKit.Nodes
  OpenBeOS.MediaKit.NodeKinds
  OpenBeOS.MediaKit.Sinks
  OpenBeOS.MediaKit.Sources

 OpenBeOS.DecoderEncoderAPI
  OpenBeOS.DecoderEncoderAPI.CurrentPractice
  OpenBeOS.DecoderEncoderAPI.OtherApproaches
  OpenBeOS.DecoderEncoderAPI.OutstandingIssues
 OpenBeOS.ExtractorWriterAPI
  OpenBeOS.ExtractorWriterAPI.CurrentPractice
  OpenBeOS.ExtractorWriterAPI.OtherApproaches
  OpenBeOS.ExtractorWriterAPI.OutstandingIssues
  OpenBeOS.ExtractorWriterAPI.ProposedAPI
  OpenBeOS.ExtractorWriterAPI.ProposedAPI.Extractor
  OpenBeOS.ExtractorWriterAPI.ProposedAPI.MediaExtractor
  OpenBeOS.ExtractorWriterAPI.ProposedAPI.MediaWriter
  OpenBeOS.ExtractorWriterAPI.ProposedAPI.Writer

UsingOpenBeOSMediaKit

Networking API

OpenBeOS NetworkKit
OpenBeOS MailKit 

Gaming API

OpenBeOS GameKit
OpenGL Kit - OpenGL

Tracker API

 OpenTracker 

Screen Saver API

OpenBeOS ScreenSaverKit
Haiku ScreenSaver - sceen saver example

Scripting

HeY - beos native scripting