Various free or low-cost spreadsheet programs are offering alternatives to market leader Microsoft Excel. Some of the programs use web-based architectures to allow users to conveniently share worksheets. One example is Calc, which is a part of the ThinkFree Office suite and which provides 300 features compatible with Excel. Adventnet offers Sheet as part of its Zoho office productivity package, while Sun Microsystems' OpenOffice offers an open-source alternative with its CALC spreadsheet, which is compatible with operating systems such as Microsoft Windows, Apple Mac OS and Linux. Another free program is SocialText's WikiCalc, which merges wiki features with spreadsheet functionalities. Google is deemed to be a potential major competitor of Microsoft as its Google Docs product continues to entice more users. Since its introduction in October 2006, Spreadsheets has been upgraded to include a presentation feature, charts, and an enhanced file-management system. Competitors are managing to cut into the market as they respond to complaints about the difficulties of sharing spreadsheets when using Microsoft Excel. Despite the emerging competition, Microsoft Excel is expected to remain the top choice among spreadsheet programs because of the drawbacks in the alternative programs. A number of the alternative spreadsheets, for instance, are in beta versions and may require IT expertise to install.