The first thing I need to point out is that topic and tag feeds are essentially just searches for future content. This is how PubSub advertises it in fact - "Search the Future!" is one of their taglines.
Normally when you use Google or MSN, you're searching for past content. So the key difference to traditional web searching is that by subscribing to searches, you automate the process and future results will come flowing to you via RSS. If you have a topic of interest that you want to keep up-to-date with, then it's much simpler to subscribe to searches on that topic and wait for results to come to you - instead of you going looking for them.
So let's get to the nitty gritty. I mentioned PubSub above, so I'll start with them. They're a relatively new kid on the search block, but unlike the others (Google, Yahoo!, etc) PubSub focuses exclusively on 'future search'. It's real simple to get started on PubSub - the homepage has a textbox where you're invited to "create a free PubSub subscription".
Say you want to keep up with the latest news and blog posts on the All Blacks rugby team (I have to use a New Zealand search!). Enter "all blacks" (with the quote marks) into PubSub and it sets up the search. The only drawback for PubSub currently is that often you have to wait a while for results to appear. e.g. it displayed this message for my "all blacks" query: "Right now, we are matching your subscription against millions of new information items as they are created." It asks me to either bookmark the page (how Web 1.0!), subscribe to the page in an RSS Aggregator, or download their browser sidebar tool. This post is all about subscribing to searches using RSS, so I'll go with option 2.
You subscribe to the feed by copying the URL into your favourite RSS Aggregator. Note: I've had problems with some PubSub feeds in Bloglines - but rather than detail it here, because it may actually be a Bloglines issue, I'll email PubSub.
I've personally found PubSub's search feeds to produce the best quality links - consistently. Which I think is a credit to their search algorithms, whatever those might be. The one thing they could improve on is the subscription process. Because the results are often not immediate, it can be confusing for the user to have to wait a few hours for things to happen.
Blogdigger and Feedster are two other blog search engines that offer topic RSS feeds. With Blogdigger, simply enter your search term and the results page has a "Subscribe to this Search" orange RSS button in the top right corner. It's the same procedure in Feedster - enter your search and look out for the orange 'XML' icon in the top right corner to subscribe to that search. Kudos to both Blogdigger and Feedster for having a simple subscription process.
del.icio.us is another great tag feeds service. Unfortunately it doesn't have a search interface (surely that's the highest priority feature on its development To Do list?), nevertheless it's easy to find the tags that interest you. Simply enter e.g. http://del.icio.us/tag/allblacks into your browser (that one currently has only 1 item in it as of this date!). Alternatively, have a browse around delicious - especially the popular page - and hunt out the tags that interest you. To subscribe to a tag, look for an orange 'RSS' icon at the bottom of the page - and as usual, save it to your favourite RSS Aggregator.
If you're a Bloglines user, then there's a good tag feed service integrated into Bloglines. Look for the Bloglines search box, enter your query, then click the "Subscribe to this search" button on the results page.
Another one I've been testing is the beta MSN Search. It has a promising piece of functionality named "Search Builder", which enables you to filter results. But I have to say MSN has produced mixed results for me. My search feed for "web 2.0" has thrown up mostly old links - sometimes downright ancient in Web terms (pre 2004 Web 2.0 conference!). Maybe this has been improved in the live version of MSN Search... In any case, once again look for an orange 'RSS' button at the bottom of the results page to subscribe to a search.
Google has a recently added 'future search' feature...of sorts. It's called Google Alerts and you can find it by doing a Google News search. Look at the bottom of the results page and you'll notice a textual message like so: "New! Get the latest news on all-blacks with Google Alerts."
If you have a Gmail account, then it gives you the option of subscribing to the search via Gmail. You can subscribe to a search on news and/or the Web (why then don't they add a link to this service on Google Web search result pages?). Apparently you can use a non-Gmail email address, but you have to sign up for a Google Account. It's a neat service, but why not add RSS functionality hmm Google?
Of course let's not forget Technorati :-) They have a keyword-tracking service called "watchlists", which you can subscribe to as RSS feeds once you've signed up as a member. Technorati recently introduced tags, but they don't offer RSS feeds for them as of this date.
Those are just some of the services available for doing tag or topic searches and subscribing to them in RSS. There are others out there too, e.g. Findory (which I signed up for today).
The general pattern for all these sites is to enter a search as you normally would, then look for an orange 'RSS' or 'XML' icon (they're interchangeable in the blogging world) in either the top-right of the page or at the bottom. Click on the orange icon and save the URL to your fave RSS Aggregator. Welcome to the Age of Topic Feeds!
]]>Workaround: After creating the search ignore the URL it says to use. Instead click on the term on the side box and then have bloglines subscribe to that page.
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