Pidgin! I switched from Digsby to Pidgin about a month ago, and I'm not missing Digsby at all (and I started off as a Digsby fanatic).
From the About Pidgin page:
Pidgin is a chat program which lets
you log in to accounts on multiple
chat networks simultaneously. This
means that you can be chatting with
friends on MSN, talking to a friend on
Google Talk, and sitting in a Yahoo
chat room all at the same time.
Pidgin runs on Windows, Linux, and
other UNIX operating systems. Looking
for Pidgin for OS X? Try Adium!
Pidgin is compatible with the
following chat networks out of the
box: AIM, ICQ, Google Talk,
Jabber/XMPP, MSN Messenger, Yahoo!,
Bonjour, Gadu-Gadu, IRC, Novell
GroupWise Messenger, QQ, Lotus
Sametime, SILC, SIMPLE, MySpaceIM, and
Zephyr. It can support many more with
plugins.
Pidgin supports many features of these
chat networks, such as file transfers,
away messages, buddy icons, custom
smilies, and typing notifications.
Numerous plugins also extend Pidgin's
functionality above and beyond the
standard features.
Pidgin integrates with the system tray
on Windows, GNOME2, and KDE 3.1.
Pidgin is under constant development.
Releases are usually frequent and
driven by user contributions, such as
bug reports and patches.
Pidgin is free and contains no ads.
All our code is open source and
licensed under the GNU General Public
License. This means you can get
Pidgin's underlying code and modify it
to suit your needs, as long as you
publish the changes you make for
everyone to benefit from as well.
It took a little playing around with setting to get it the way I like, but I haven't looked back yet. And big bonus, free AND open source!
Edit: Whoops, missed the Twitter part of the question. Without further ado, I give you DownloadSquad's How to Add Twitter and Facebook IM Support to Pidgin for Windows!