You could write a book on this. Heck, I probably should - I've done it a few times :)

A short list (but by no means anywhere near complete!):

- Find as many blogs in your niche as possible. Pitch them properly (a lot could be written on that point alone) DON'T JUST GO FOR THE BIG ONES! The smaller blogs are more likely to link to you if you're friendly to them and develop rapport.. I run blogs with over 10,000 subscribers and I love helping people who are FRIENDLY and GENUINE.

- Use your social network.. you've been building one up, right? Make sure all your Twitter and Facebook followers know about what you're doing. Lean on your Linked In contacts.

- Stumbleupon advertising (if appropriate, 5 cents a visitor). Adwords advertising (if appropriate).

- Find places where users of competing applications gather (forums, Google Groups, etc) and work your way into their attention zone.

- See if there's a sub-Reddit that's specifically for your niche. Find people to charm there, post ancillary links regarding your app, etc. Don't over-do it.

- Post it on HR (as someone said above)

- Find your way in to interviews, podcasts, etc. A lot of content providers are dying for more content - you might make a great interviewee. The media is less opaque than it seems.

- Go to events! Make sure you have an elevator pitch. Get excited. Wear schwag featuring your logo, etc, if you want to. Don't just focus on the big-wigs - get anyone who might find your service useful excited.

- Does your design rock? Get on to the "CSS design", and "Web design" show case type sites. There are hundreds of them around. Not amazing exposure, but the more links the better and any one of your visitors might turn in to a serious contact.

- Start your own blog for your company / startup. Make it really interesting. Be candid. Show off new features. Show off stuff you're working on. Show off your team or your technology. Build up your own tribe of followers. They will make all the difference when it comes to saving you on del.icio.us, Digg, Reddit, and so forth.

- Make sure you stay on top of your e-mail. Customers might test you with e-mails - responding quickly and completely can make the difference between sales and no sales - or life and death with a startup.

- Find ancillary reasons to get your service mentioned in blog posts and tutorials. For example, if your startup is an RSS mashup generator of some sort, you need to have tutorials out there that recommend your service. Get those tutorials and posts on to Reddit, Hacker News, Digg, etc.

- If people write about your site, write tutorials that mention you, etc, PROMOTE THAT CONTENT EVEN IF IT'S NOT YOURS! Get people reading stuff that's about you - not by you!

- Remember that bigger sites like TechCrunch and ReadWriteWeb (if applicable to your sector) love exclusives. Don't bother mass pitching those - focus on one, whichever you can get best rapport with, and offer an exclusive. Your product needs to be AWESOME for this to work though.

- Follow a search.twitter.com search on terms related to your service (and even the name of your service) .. get in touch with people who might be interested, respond to all comments about your service.

- Write a bog standard press release and submit through the standard channels. This will not help much but at least your company name/service name will end up with a ton of results in Google - this can help you look bigger than you are. You /may/ even get some offline coverage if the press release is actually kinda good (but not too crazy). It's cheap to do this.

- Build ancillary "fun" services that tie into your main web app. Something fun, free, perhaps something that you can relate to sites people find interesting, such as Twitter. Let's say your main service is an online graphics editor. Perhaps you could create a separate site where people can create avatars for Twitter / Facebook from a small set of templates.. separate project but promoting the first.

- Hustle, hustle, hustle! Make sure you know as soon as someone blogs about your service. Follow Google Blog Searches, etc. Keep Googling. Get commenting on blogs (not in a spammy way - just get your name and service out there). If someone needs to do something your service offers, you need to be there!

I believe Jason Calacanis wrote an interesting piece on doing PR for a startup recently. Find that article and read it - I recall it was very good.

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BTW, if anyone thinks I might be able to turn this into a good book, resource site, or similar, vote this up and I might give it a try! :)