[halfastack]

Thunderbird is, hands down, my most used desktop app, if we're not counting IDEs and editors. Definitely would recommend; Thunderbird is awesome.

[neilsimp1]

Honestly, I thought I liked Outlook on Windows until I gave Thunderbird a try. It's just so much faster.

[kgwxd]

Not counting browsers?

[timbit42]

Maybe they don't surf the web all day at work.

[kgwxd]

Reference; research; testing and debugging web sites in the 5 major desktop browsers; communication and data entry apps required for work that have a horrible desktop client, or no desktop client at all; etc. There's a lot of useful, work related things browsers provide access to.