Still the best, at least for now

While Digsby remains tied to Windows and non-portable (at least, not truly portable and self contained).

I need portable. I need multi-platform. I need meanwhile (Sametime) support, in addition to jabber (google talk), icq, irc, yahoo and msn.
Pidgin is still the only IM that offers me all these things, with an attractive, easy to use interface, and all for free (unlike Trillian).

PidginPortable is presently at version 2.4. This may or may not be better than previous versions - but assuming you have or will download that version and what to fix the one known bug that matters, or get it working with Sametime, then this is how you can do it.

Buddy List Search Bug:
This is bothersome. You have a huge list of buddies, spread across multiple groups and multiple IM protocols, and you can't be bothered scrolling through the list, expanding groups and visually searching for buddies until your eyes bleed.
Up until PidginPortable 2.4 (true for non-portable flavour too), you just started typing the first few letters of the contacts name and pidgin did the rest. However, since the inclusion of the latest GTK+ libraries, this doesn't work - and pidgin just stops the search after you type the first letter. Downer.

Solution:
Download GTK+ 2.12.6 and copy bin/libgdk-win32-2.0-0.dll into PidginPortabl/App/GTK/bin (replacing the one that's already there).
Restart PidginPortable and the problem will be solved.

Sametime support:
So you work for a large blue corporation or some company that buys their IM/collaboration solution from them.
Older versions of Pidgin (Gaim) didn't support Sametime out of the box - you had to install the meanwhile library first. Now, meanwhile is included, but Sametime 7.5 will check your client version and reject Pidgin because it looks like an old Sametime version (3.x).

Here's how to fool Sametime:
Look for accounts.xml (for PidginPortable it's in Data/settings/.purple)
Look the following settings:

<settings>
<setting name='client_major' type='int'>30</setting>
<setting name='server' type='string'>messaging.yourcompany.com</setting>
<setting name='fake_client_id' type='bool'>1</setting>
<setting name='port' type='int'>1533</setting>
<setting name='client_minor' type='int'>6511</setting>
<setting name='force_login' type='bool'>1</setting>
<setting name='client_id_val' type='int'>4098</setting>
</settings>

The 'server' and 'port' lines will be there already from when you added the account, but you'll have to add the 'client_major', 'client_minor' and 'client_id_val' lines.
These three lines will tell the Sametime server what it wants to hear when it queries the pidgin client.
Another problem solved.

Thanks to khaytsus for raising the ticket for the Buddy List Search GTK+ bug and thanks to datallah who took ownership and of course witmaster who posted the downgrade workaround solution.
Thanks to jno and siege for the Sametime workaround.

Kudos to all who work on Pidgin to make it better and keep it relevant, despite stiff competition from the likes of Miranda and Digsby.