Ok well we had a little from linux/unix people, so I'll add into that bunch - however most of what I am going to list is cross platform by personal preference:
My favorite editor so far (next to vim in a shell) was recommended by El_Spectre:
http://www.activestate.com/komodo_edit/
I prefer this media manager/mp3 player manager to itunes etc. mainly due to it being built on the gecko engine and neat as hell plugins:
http://getsongbird.com/
Calendaring via Sunbird (Or the Lightning extension for Thunderbird) this also works with Google Calendars with a few small tweaks:
http://www.mozilla.org/projects/calendar/sunbird/
http://www.mozilla.org/projects/calendar/lightning/
Cross platform control of multiple machines via one keyboard/mouse (assuming they all have their own monitor):
http://code.google.com/p/synergy-plus/
Not sure if anyone has mentioned it yet, but I am a big fan of Pidgin, and it's smaller console-based finch. I like it mainly for the killer OTR encryption plugin (stands for Off The Record) that provides excellent in transit ecryption between clients. Finch's only drawback is I had to write my own plugin for OTR encryption to be used. I hope this changes soon. Granted Pidgin isn't exactly obscure, but the OTR plugin isn't well known.
http://www.pidgin.im/
http://www.cypherpunks.ca/otr/
This one is cross platform, only insomuch as you'd better be a ubuntu 8.10 user and newer to easily install (though you can compile from source) and is one of the easiest and best DVD ripping tools out there, IMO:
http://handbrake.fr/
Also to use with ubuntu you'll need to follow these direction to get dvd encryption working (other platforms have similar how-to's all over google.):
http://lifehacker.com/350015/enable-dvd-playback-in-ubuntu-in-two-commands
Cross platform free video editing (via a sign up):
http://jahshaka.org/
I like many different bittorrent clients, but these three stand out as the best of easy and functional:
http://deluge-torrent.org/
http://www.transmissionbt.com/
http://www.vuze.com/app
Then again, I mainly just use rtorrent. That's found here or in your favorite linux repo:
http://libtorrent.rakshasa.no/
Not obscure, but always needs to be mentioned due to awesomeness:
http://www.videolan.org/vlc/
Also, There are several good options for encryption that are cross platform, but many are not free. TrueCrypt is very effective, and on a windows machine supports full disck encryption with an option to install a hidden OS accessed by a different passkey (for "under duress" situations you need to boot a clean/different OS):
http://www.truecrypt.org/
Full disk encryption on my *nix boxes is done via LUKS:
http://en.wikipedia.org/wiki/LUKS
All of the above is cross platform. Now at home I only run *nix (opensolaris and ubuntu) I'll get into the very cool platform specific for those:
SSHerminator:
http://lifehacker.com/5135130/ssherminator-splits-ssh-terminal-windows-into-panes
(This is included in opensolaris builds newer than nv105)
While OpenOffice is very good for full featured office suites, often my machines only need lightweight installs so I use these:
http://www.abisource.com/ (This is cross platform)
http://www.gnome.org/gnumeric
And for my accounting needs:
http://www.gnucash.org/
I've converted my small recording studio to ubuntu studio edition and use the following:
http://ardour.org/
http://www.rosegardenmusic.com/
VM/Virtualization via Sun's VirtualBox:
http://www.virtualbox.org/
On a side note, for those who love VIM, you can make firefox act like vim (this serves no good purpose I can find except to be a neat novelty):
http://www.developernotes.com/archive/2008/06/30/vim-firefox-vimperator.aspx