.avi files can be opened with a variety of programs including:

• Windows Media Player (default for many Windows PCs, but may require additional codecs for some files)
• VLC Media Player (free, open source, cross-platform, supports almost all media formats)
• PotPlayer (for Windows, supports many codecs and subtitles)
• MiniTool MovieMaker (plays and edits AVI files)
• RealPlayer (supports multiple formats and streaming)
• MX Player or BS Player for Android devices

Among these, VLC Media Player is one of the most popular and versatile choices for opening .avi files across different platforms due to its broad codec support and ease of use.