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What a fantastic couple of years it has been for Google. From being merely the best search engine, it has spurted ahead and elbowed Microsoft aside as the leader in personal computing innovation. I've been searching with Google forever (or at least since it overtook AltaVista and Inktomi), but recently I've started using Google Mail, too, routing email from my other accounts through its web interface.

I didn't really like the Google Mail interface at first - presenting everything in one big list felt like a step backwards. But once I grasped filters and labels, I was able to build a far more sophisticated filing system for my mail, because a single message can have several labels and thus belong to several "virtual mailboxes".

This principle of labelling or tagging things is becoming enormously important nowadays. Web 2.0 applications make much use of tagging as an aid to navigation. For example, two sites I use a lot - Flickr and LibraryThing - both encourage you to tag all your pictures/books in ways that accurately describe their contents. I've found that sensible tagging on Flickr can increase the views your pictures attract, and the "tag cloud" (also employed by LibraryThing) is perhaps the only really useful innovation in user-interface design for years. For those unfamiliar with it, you get a screenful of the tags that you've applied, arranged as a random cloud in a typeface whose size is proportional to the frequency with which that tag occurs. At a glance, you can see which topics are most popular, and a single click on a tag brings up all

A similar concept of tagging lies behind Tim Berners-Lee's next big idea, the semantic web, and with the increasing use of XML as an output format that looks as if it might finally become a reality. In brief, the idea involves tagging documents and web pages to indicate their subject matter (for example, this column might be tagged ) using XML-based markup languages, so documents can be searched for by content. It hasn't taken off so far because the web remains stubbornly wedded to plain, old fixed-tag HTML. For all the sniping from the Anything-But-Microsoft brigade, the ISO standardisation of Microsoft's OOXML file formats can only be a good thing by accelerating the spread of XML, and until XML becomes ubiquitous a semantic web can't deliver its promised benefits.

But in any case, there's a further question hanging over the semantic web idea, which is who does the work of tagging? It's all very well having web browsers that understand XML and can peer inside files to read the content tags, but who'll put those content tags there in the first place? The answer has to be the original creator, and it has to be made so easy that it's a no-brainer to do it rather than skip it.

Writing XML code doesn't fall into that category. All tagging systems are also plagued by misspelling. If I invent a tag "Poggioni", then either the parsing system needs to be smart enough to guess that "poggioni" and even "Pogionni" were probably intended to refer to the same tag, or else the software needs to present me with a menu of my own tags to select from rather than (mis)typing them every time. Unless one of these conditions pertains, you'll get orphaned data, not retrievable solely because of misspelled tags.

Flickr has this quite well sorted, because it invites you to tag your pictures at every stage during their uploading. It only falls down in one respect: on the Flickr page for each individual picture, the "Add a tag" dialog does indeed offer a link called "Choose from your tags", but this ability is not offered when you first add tags during upload, which is where it's needed.