With the messy demise of PubSub, I've been looking for a decent keyword and topic subscription service to replace it. ZapTXT could very well meet my requirements, except for the odd fact that it doesn't output in RSS (more on that below). You can currently only receive alerts as an email, IM or SMS.
Other than being sans RSS, ZapTXT's latest release does offer a range of good features. You can now create a monitoring task across multiple feeds or URLs for specific keywords - they call this "task monitoring", as opposed to per feed monitoring. It also enables you to refine your feedset for any given keyword search, there's a Zap It! bookmarklet for your browser, a publisher 'chicklet' to allow bloggers to let their users receive notifications on interesting topics, nice OPML import/export options, a tag based system around feeds, and more powerful search (for example create a ZapTask using your trusted feeds with Apple, Cisco, Microsoft AND upgrade, downgrade, merger as keywords. You'll get an alert every time any of these companies make the headlines for an upgrade, downgrade or merger).
I asked ZapTXT's Sameer Patel where is the RSS feed output option? He told me "we're in testing at the moment on that so [it] shouldn't be too long."
All in all, I really like ZapTXT's ability to set up keyword searches and modify the 'tasks' in a number of ways. It really needs to output in RSS though, because that's the lingua franca of information management. In terms of the competition, with PubSub's demise the leaders in this space are now Technorati and Feedster. I also really like Topix, which does other things besides topic searches. Others I'm keeping my eye on include FeedRinse (nb: I've been an advisor to them in the recent past), Bloglines/Ask, Google Blog Search, Findory - and all the ones I mentioned in my post from February 2005 How To Subscribe To Topic Or Tag RSS feeds.