Thunderbird is a fully-featured client for email, newsgroup, news feeds, and text chat that is easy to set up and customize. It supports the standard IMAP and POP3 mail protocols out of the box with additional features available via Mozilla's popular add-on system.
Writer is a word processor application for authoring text documents. An alternative to Microsoft Word, it can be used to open and edit doc and docx email attachments.
Calc is a spreadsheet app for collecting, storing, and analyzing data. Similar to Excel, it works with xls and xlsx spreadsheets commonly attached to emails containing report data.
Impress is a multimedia presentation program like PowerPoint for viewing and playing attached ppt and pptx presentations.
Sumatra PDF is a fast and lightweight PDF document viewer for opening and printing PDF attachments. It also supports the less common eBook and comic file formats.
7-Zip is a file archiver for extracting all common archives including ZIPs, RARs, and 7z files. It is helpful as emails are often sent with compressed attachments to reduce their size, the number of files sent, and to avoid email filters.