REAPER is a powerful but sensible Windows application that lets you record, arrange, edit, and render multi-track waveform audio. It provides an extensive set of features, but is a very small and lightweight application (the installer is less than 1 megabyte, and includes many effects and a sample project).
REAPER supports ASIO, Kernel Streaming, WaveOut, and DirectSound for playback and recording. It reads WAV, OGG, and MP3 files, and records WAV files. You can arrange any number of items in any number of tracks and use audio processing plug-ins (DirectX and Jesusonic). REAPER also supports volume, pan controls and envelopes per track, multi-layer undo/redo, and user creatable color themes.

Basic features:

- Portable - supports running from USB keys or other removable media
- 64 bit audio engine
- Excellent low-latency performance
- Multiprocessor capable
- Direct multi-track recording to many formats including WAV/BWF/W64, AIFF, WavPack, FLAC, OGG, and MIDI.
- Extremely flexible routing
- Fast, tool-less editing
- Supports a wide range of hardware (nearly any audio interface, outboard hardware, many control surfaces)
- Support for VST, VSTi, DX, DXi effects
- ReaPlugs: high quality 64 bit effect suite
- Tightly coded - installer is just over 2MB

Editing features:

- Tool-less mouse interface -- spend less time clicking
- Drag and drop files to instantly import them into a project
- Support for mixing any combination of file type/samplerate/bit depth on each track
- Easily split, move, and resize items
- Each item has easily manipulated fades and volume
- Tab to transient support
- Configurable and editable automatic crossfading of overlapping items
- Per-item pitch shift and time stretch
- Arbitrary item grouping
- Markers and envelopes can be moved in logical sync with editing operations
- Ripple editing - moving/deletion of items can optionally affect later items
- Multiple tempos and time signatures per project
- Ability to define and edit project via regions
- Automation envelopes

Changes:

* Automation lanes: fixed possible crash when removing FX parameter envelopes
* CD burning: fixes to native XP mode track divisions
* Extensions API: GetSetMediaItemInfo() fixes for fade shape settings
* Fade drawing: when using full-area filling, fixed drawing when zoomed in
* FX: per-FX buggy plugin compatibility mode (assumes plugin is not threadsafe, needs constant precleaned buffers, etc)
* Hardware outputs: post-fader track outputs obey track polarity control
* MIDI editor: show notes pressed as long as the input note is pressed
* OSX: improved drag/drop support
* OSX: support for control+alt+drag to render to new file (drag to desktop, finder, sampler, etc)
* OSX: CD burning support
* OSX: REX support
* OSX: special casing for POD Farm VST misreporting view type (VST 2.4 should be composited view)
* OSX: fix for some AUs that do not export automatable parameters until after audio streams are initialized
* OSX: added correct file locking to prevent writing files open for read
* Pencil mode: fixed fade/autofade settings for newly drawn media items
* ReaControlMIDI: support for raw mode (0-127 access to all CC messages)
* ReaControlMIDI: envelopes and TCP knobs report the same value as the slider (0-127, 0-16383, or on/off)
* ReaControlMIDI: support for storing/sending sysex dumps up to 64KB
* ReaScript: run Python or Perl scripts that call Reaper API functions
* ReaScript: assign reascripts to key commands or toolbar buttons
* VST bridging: optimized plug-ins that call for tempo information
* VST bridging/firewalling: per plug-in option to embed UI (not as compatible, less crash-resilient)
* VST: safer querying of text strings from certain plug-ins
* Win32: async file reading now has more grace on error
* Win32: file locking improvements (no longer allow writing to media files that are open and online in REAPER, etc)
* x64: VST bridging performance improvements, ReaRoute performance improvements
* x64: will now use same config path as x86 (unless appdata/REAPER64 is valid)
* x64: REX support (via bridging)
* x64: fix for certain (broken) ASIO drivers