Lotus 1-2-3 was regarded as the "killer app" of the IBM PC in its early days, just as VisiCalc was for the Apple II platform. Both were spreadsheets which used a similar interface, still familiar to modern spreadsheet users, consisting of a matrix of rows and columns where the columns have letters and the rows have numbers, and a cell can be addressed by an address such as "B2".
After years of dominance of the spreadsheet market for PC/MS-DOS, 1-2-3 failed to successfully move over to the Windows platform, and was eclipsed by Microsoft Excel. Making 1-2-3 part of a new "Lotus SmartSuite" didn't help bring it back to prominence. When 1-2-3 was finally officially discontinued in 2013 (by IBM, which now owns Lotus), the general reaction was "What, they still made 1-2-3 until now? I thought it died a long time ago."
The original 1-2-3 files had a .wks extension. Version 2.0 originally used a .wk1 extension, but that was soon changed to .wk2, and from then on the number in the extension corresponded to the program version number. Lotus Symphony (the '80s version, not the unrelated 2000s version which used a variant of Open Office format) used a similar file format.
End of support
IBM officially ceased its support for Lotus 123 on 30 September 2014 (see the announcement here). This also applies to its SmartSuite and Organizer products.
Support by modern spreadsheet software
Support of the Lotus 1-2-3 formats has shown a steady decline over recent years. Although older versions of Microsoft Excel were able to read (some of?) these formats, Excel 2010 doesn't support any of them anymore, as shown in this overview of unsupported formats on Microsoft's website. Meanwhile, OpenOffice only supports the old (.wk1, .wks and .123) versions of the format (source here). No specific info could be found on LibreOffice, but the situation is probably identical to OpenOffice.
- Version 2.0 (WK1) samples
- Version 3.0 (WK3) samples
- ZIP file on Lotus FTP site with some WK1 and WK3 files
- Tips for Importing Lotus 1-2-3 Files to Excel
- Interfacing with 1-2-3 files in SAS/ACCESS
- Lotus 1-2-3 converter (commercial software with free trial)