Feanor

None.
I've been trying the main 3 free antivirus, AVG, AVAST and ANTIVIR. Now, AVG version 8 doesn't even install correctly on Win2K (scheduler and updater are disabled), ANTIVIR sucks a lot of resources, takes forever to open dialogs and gives me a BSOD from time to time (probably only when it finds some other software running, do not know which), AVAST is the most complicated antivirus ever, with several resident modules for monitoring different activities, a skinnable GUI, options distributed in "hard to find" dialogs here and here. On top AVAST misses heuristics besides the mail module.
At the end, given the catastrofic failures of AVG and ANTIVIR I am using AVAST, with the bare minimum of modules installed but thinking of living without any "on access" scanner and use CLAMAV (ported to Win) from time to time.

Edit: did I say AVAST? So far I got this interesting "features":
1. scanning my 170MB Thunderbird mailbox it generates 2 or 3 120MB temp files, if I stop the scanning before it ends those temp files are left on the disk in the _avast_ directory. It seems also that after creating those files AVAST doesn't actually do anything, skipping the content. On a side note, ClamAV does the same with temporary files if you "cancel" before completion.
2. there is a feature to block (it asks for authorisation) writing of executable files, unfortunately it is triggered also by directories whose name is "mario.com" and some software installers create such folders, for example TopStyle. An alert is risen any time you access a dir if some subdir has such names.
3. automatic updates of the virus signatures can cripple the scanning engine.

In case your mail client doens't have an embedded spam filter, or it doens't work well enough, you can use this very nice FREE (modified GPL) tool, courtesy of a german guy, Michel Krämer, that sits between the mail client and the Internet and filters all your spam.
Grab it here: http://www.spamihilator.com/
The program works with almost every E-Mail client, such as Outlook 2000/XP/2003/Express, Eudora, Mozilla Thunderbird, IncrediMail, Pegasus Mail, Phoenix Mail, Opera, etc.

Maybe you are interested in these articles a friend of mine is writing on dotnetslackers.com:
ASP.NET MVC Framework
His english blog is linked in the sidebar.

For those of you that are too young to remember the old good times of the Internet maybe it doens't mean much. Anyway this blog post is VERY interesting to learn what happened after Netscape (the company) collapsed and was acquired by America On Line.
http://weblogs.mozillazine.org/asa/archives/2007/12/its_about_time.html

On a side note: this is also related to the main problem with the true inner nature of the Web, the business model for anything Web-related and the continuos struggle between the marketing strategists and the technicians, who often have different when not opposite visions.

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Per quelli di voi che sono troppo giovani per ricordare i bei vecchi tempi di Internet forse non significa granchè. Comunque questo post è MOLTO interessante per capire cosa successe dopo che Netscape (la azienda) crollò e fu acquisita da America On Line.
http://weblogs.mozillazine.org/asa/archives/2007/12/its_about_time.html

Nota a margine: questo è anche collegato al principale problema della vera natura del Web, il modello di business per qualsiasi cosa Web e la continua lotta tra gli strateghi del marketing e i tecnici, che spesso hanno differenti visioni, quando non contrapposte.

There have been some discussions on the Web about this:
http://www.opera.com/pressreleases/en/2007/12/13/

I've seen a lot of bad comments so far like my friend Simone (.Net developer mostly) on his blog here:
http://codeclimber.net.nz/archive/2007/12/17/Sick-of-Opera.aspx
Or Asa (from Mozilla corp.) here:
http://weblogs.mozillazine.org/asa/archives/2007/12/opera_calls_for.html

I've got mixed ideas. First of all this is a never ending issue about Microsoft being so dominant on the software market that they can make the rules and there isn't any real "competitor", other software makers can either play by Microsoft rules or live in the little thiny spaces Microsoft hasn't colonized yet. Once upon a time there was Netscape and Java then Microsoft finally decided Internet and Web Applications/Services could be real business so they first integrated Internet Explorer in the operating system to crush Netscape then developed the .Net technology to crush Java. There have been some years when all the developers thought there was no point in considering anything else but Microsoft as standard, till from the ashes of Netscape resurrected Phoenix now called Firefox, backed by a big player like Google. People at Google so far tried their best to not confront Microsoft directly, yet Microsoft launched MSN with search engine and the "Live" services that mimic Google main business. Everybody knows about the EU ordering Microsoft to separate their Media Player from the O.S. but that came when the war about multimedia/video players was already almost over. Who thinks Real Player can be a competitor now? Adobe with Flash and related technologies on the RIA applications are "doubled" by SilverLight. I don't know if Adobe sees Microsoft as a competitor or what, given they have exaclty the same business model based on license fees. Opera's action to me seems just another round. Since I don't make a living on Microsoft products or related business, I welcome what ever initiative that potentially can make things better for the whole IT and ultimately for the users. The only flaw I can see here is that nothing could stop Microsoft so far either make them change their course of actions, so I guess the whole thing is a little pointless. Opera has got less than 1% of the browsers "market", it is extremely improbable they can get much more than some sort of agreement by sueing Microsoft. When AOL acquired Netscape they sued MS, then agreed for some million dollars and quit Netscape. If we consider the million developers who work on Microsoft technologies, they see Opera mostly like a pain in the ass, so no support can come from that direction. The Open Source world is too busy in their own internal civil wars and watching their bellybuttons to make any difference.

Time will tell. In my opinion it isn't time any more for David against Goliath, so I am not positive. Mostly because Opera doens't have any support out of its own small community of fans. Besides, I guess even the Web in itself is changing and so is the "browser" concept.