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Document Reader

OpenOffice – Has most of the same features as Microsoft Office however not quite as fancy as an interface. Can read and write Microsoft office documents. Why pay money for office when you can use this for free.

Tip – Change the save settings to automatically save documents in .doc format, if you save files in the open office format users with Microsoft word might not be able to open it.

Google Docs – Keeps all your documents online, and can now be used offline with google gears.

Media Players

VLC Media Player – This lightweight player plays every video file I have thrown at it except for real media files, it can also play mp3, .iso, and DVD’s, especially a must have if you download movies.

Songbird – Good replacement for iTunes.

Web Browsers

Firefox – Constantly updated so any bugs or security threats are usually fixed fast, and has tons of extensions to choose from.

Opera – Another browser that is my favorite right out of the box, if you are not into messing around with extensions this is for you.

Email

Thunderbird – Good replacement for mail or outlook.

Image Editing

Gimp – Photoshop replacement.

Picasa – Much like iPhoto a great for collecting your photos, doing basic edits, tags, and  backing up your photos online.

File Sharing

Frostwire – p2p, torrents, from the creators of limewire.

Instant Messaging

Skype – IM, Video, Audio chat, can also be used to call regular phones.

Tip – Not a native app but I personally like living in a cloud, Meebo is a great online IM client that connect to just about anything.

FTP

Filezilla

I also like FireFTP which is a firefox extention.

Other Programs

Google Earth – Offers maps and satellite images for complex or pinpointed regional searches.

Logmein – Control your computer remotely when your not at home.

Celtx – Combines full-feature scriptwriting with pre-production support and also enables online collaboration.

Games

Wormux – Just like the classic worms game but free and cross platform.

If you know of any other cool free cross platform apps, let me know about them!