EFnet has #mtgplay for finding opponents.

Apprentice is great because it is free and gives
you all the cards by default. However, it is also
very unreliable and buggy. Be aware that there
is loads of Apprentice cheating out there, even
though I assume majority of your potential opponents
will not be cheating.

There is a new online environment for playing
Magic, supported and launched directly by Wizards
of the Coast. It is called Magic Online, or MODO.
Read about it somewhere on www.wizards.com/magic/
if you haven't managed to catch the Magic world
headlines during the past weeks.

Magic Online beats Apprentice stone dead in terms
of technical and visual greatness. However, it
does not give you all the card pool at once: you
have to accumulate cards from boosters just like
in reality. Currently MODO is in beta testing
phase. Limited number of accounts are available
and it is completely free for the beta period.
However, when it is evetually released as a full
version, the online boosters are going to cost
the same as real boosters.

Well, that's about it in a nutshell.

Yrs
Thaurwylth
Team Extreme