Wow, these are some great suggestions. Thanks for the support.
The Phoenix:
Use a forum software that offers rss feeds on your site (then it can hit the aggregators)
Done; Phorum has RSS feeds for every topic, and I also have a news feed. Probably should submit them to Technorati or something.
Add a pitch regarding its speed and memory use. The library does amazing things, does it fast, and does it in little memory. Many are attracted to those, and your site doesn't totally sell that.
I'm not sure how I can do this in an honest manner. HTML Purifier is reasonably fast (and some added caching in this release makes it even faster), but there's nothing I can compare it to: all the other filter libraries don't go to the same depth and consequently are worthless benchmarks (Though I have never tried it for myself, I am 90% certain that HTML Purifier will be slower than any of the inferior products on the market).
Start a Sourceforge (or similar) project page. Its free PR5+ publicity that can attract users just browsing.
Done. Haven't really pimped it out yet, maybe the description's not good enough?
http://sourceforge.net/project/htmlpurifier
Pimp the Freshmeat page for it. Make sure every release is announced there, ask for votes, and man the comments section regularly. Its more PR5+ publicity for little effort.
Freshmeat is great: I get a spike of referrals from them whenever I post a new version up. Unfortunately, no one ever votes for it or makes comments: I'll try asking next time.
http://freshmeat.net/projects/htmlpurifier/
(Side note: for a while Freshmeat came up higher on the Google search for HTML Purifier then my website. That's funny)
Try to break into major projects. Offer to do the leg work to implement it into next-gen projects (like phpbb3, drupal, etc), so that you get more testers, more exposure, and more clout. As ugly as phpbb is, its a paradise for free testing and attacks.
We have plugins for ModX and Drupal, and a Wordpress plugin coming down the pipes. I'll try reviving this effort and getting out plugins for more open-source projects.
Podcasts are popular, so are php magazines, and php conferences. If you have a conference near you, offer to give a presentation. It'll get you free admission, experience with public speaking, good face time with potential users (many of whom will provide top-notch testing), and first-implementers willing to try it out.
I was going to have an interview with Pro:PHP podcast a month ago... let's see if they're still interested. I don't think I'm going to be going to a conference any time soon though... that would be nice though.
The Ninja Space Goat:
I will definitely make a note to blog about HTML Purifier. Nobody reads my blog, but it will make me feel special. We do at least have a PageRank of 4 though, so that may help you.
Thank you! Ever little bit helps.
astions:
And that's probably about all it would take to rank in the top ten for that search term.
It already is.

But it's only sixth, I think, so more links will help get it up.
PatrikG:
Will do so. Chris Shiflett has mentioned HTML Purifier on his blog before, and I got a load of hits from his website, so I'll ask him whether or not he'd like to write a full piece on HTML Purifier.
Maugrim:
Whenever the release takes place I'll draw attention to it on my blog. I get on Planet-PHP and PHPDeveloper.org so it will get a wide PHP-oriented audience. You can also submit a summary of the release to PHPDeveloper.org. It distributes news, not just summarised blog posts.
Thank you! Planet-PHP is a great aggregator, and I'm sure that'll get a wide audience.
Oren:
The Phoenix pretty much summed it up. I do have an old site with PR4 though, and right now I'm considering bringing it back to life. Whether it happens or not, I'm gonna add there links to Swift and HTML Purifier very soon. I also might put links to both on my friend's site (PR1 I think) if that's ok with him. I also promised you a quote so you can put it on your site, but right now there is no point of doing that... maybe later on (if you want to know why, PM me)
Thank you! (I seem to be repeating myself) I'll PM you about the quote: any user testimony is appreciated.
Once again, thank you everybody who offered to link to HTML Purifier on your websites/blogs. Any further suggestions?