Search is the most widely used application on the internet and while it has come a long way from the yahoo index or alta vista search there is still vast room for improvement in the search field. One area that at least needs to be distinguished is subscriptive search. I know, you're saying subscriptive isn't really a word, but it sounds so much better to me than "persistent search". You see, I think there are really two primary search use cases.
There is the one we are all familiar with and use Google for daily. That is ad hoc search. Ad hoc search is used when we need to find something. It may be something we used to know but have since forgotten, it may be what the weather is going to be like somewhere we're going to, it may be figuring out what the GDP of Austria was in 2006. These are tasks that we don't repeat day in and day out. We perform the search when it is warranted, often looking through only the first page of search results, plucking out the desired information, and are on our way. We don't spend a lot of time with the data, we find it, consume it, and move on.
The second form of search is subscriptive. There's that word again. Ok, we can call it topical subscription but that is so long and unwieldy. Subscriptive search is used when we need to know what's going on around a topic that we're highly interested in, and please keep us up to date. I don't want to go and re-search, I want you to tell me when there's something important and why I should care. A number of weather services have popped up to provide this in the specific information vertical of weather forecasting. I always want to know what the weather is going to be like where I live. Don't make me go search for it every day, just deliver it to my desktop or email me and definitely be proactive about letting me know when some dire weather pattern is about to descend on me. The same is true for a number of common areas of interest: sports scores, movie times, top news stories, and most of the things that you can customize your My Yahoo! page today.
The thing that is missing is in the Long Tail. If I have a passionate interest in antique lawn mowers I can find a couple of blogs or I can continuously search, or I can use something like Google Alerts. None of these get me what I would really like though, which is a page that I can go to that pulls any new posts from blogs or news (with rankings that are personalized to my specific interests), shows me about-to-expire auctions on ebay for those mowers, and whether there are any upcoming shows around the country. It would also allow me to tag the content I wanted to save so I could review it later. I could also expose my page out so that others could find it and use the knowledge I gained in building my page and just read that page, or use it to build out their own page with mine as a template.
This would essentially allow anyone to publish their own zine on the areas that interest them most including multiple content sources, and would be perhaps the 2nd stage (after blogs) in really creating an n-to-n publishing model that will fill a key role in next generation content distribution and discovery. Hey, what a great idea for a startup!