Here's some free (open source) software that I think are pretty cool. They might not be as well known as some other software, but they're worth giving a try and yes, they're cross-platform :

LyX - LyX is a document processor that encourages an approach to writing based on the structure of your documents (WYSIWYM), and not simply their appearance(WYSIWYG).

LyX combines the power and flexibility of TeX/LaTeX with the ease of use of a graphical interface. This results in world-class support for creation of mathematical content (via a fully integrated equation editor) and structured documents like academic articles, theses, and books. In addition, staples of scientific authoring such as reference list and index creation come standard. But you can also use LyX to create a letter or a novel or a theatre play or film script. A broad array of ready, well-designed document layouts are built in.

LyX is for people who want their writing to look great, right out of the box. No more endless tinkering with formatting details, “finger painting” font attributes or futzing around with page boundaries. You just write. On screen, LyX looks like any word processor; its printed output — or richly cross-referenced PDF, just as readily produced — looks like nothing else. LyX is released under a Free Software / Open Source lisence, runs on Linux/Unix, Windows, and Mac OS X, and is available in several languages.
(Taken from the LyX website.)


WXDownloader Fast - WXDownload Fast (also known as wxDFast) is an open source download manager. It is multi-platform and builds on Windows(2k,XP), Linux and Mac OS X(binary still not available). Besides that, it is a multi-threaded download manager. This means that it can split a file into several pieces and download the pieces simultaneously.
Created in C++ using the wxWidgets(wxWindows) library.
(Taken from the WXDownload Fast website)


NotePad++ - Notepad++ is a free (as in "free speech" and also as in "free beer") source code editor and Notepad replacement that supports several languages. Running in the MS Windows environment, its use is governed by GPL Licence.
Based on a powerful editing component Scintilla, Notepad++ is written in C++ and uses pure Win32 API and STL which ensures a higher execution speed and smaller program size.
(Taken from the NotePad++ website.)


RSSOwl - RSSOwl is a news aggregator for RSS and Atom News feeds. It is written in Java, and uses SWT as a widget toolkit to allow it to fit in with the look and feel of different operating systems while remaining cross-platform. Released under the Eclipse Public License, RSSOwl is free software.
(Taken from the RSSOwl website.)


SpeedCrunchl - SpeedCrunch is a fast, high precision and powerful desktop calculator. Available for Linux, Microsoft Windows, and Apple Mac OS X.

It's features include:
1. History and results on a scrollable display.
2. Up to 50 decimal precisions.
3. Unlimited variable storage.
4. Intelligent automatic completion.
5. Fully usable from the keyboard.
6. More than 50 built-in math functions.
7. Optional virtual keypad to be used with a mouse.
8. On-the-fly and selection calculation.
9. Customizable appearance.
10. Syntax highlighting and parentheses matching.
(Taken from the SpeedCrunch website.)


VLC - VLC media player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, mp3, ogg, ...) as well as DVDs, VCDs, and various streaming protocols.
It can also be used as a server to stream in unicast or multicast in IPv4 or IPv6 on a high-bandwidth network.
It doesn't need any external codec or program to work.
(Taken from the VLC website.)

Thanks for reading!