Promoting a free web app can be very easy if you know how.
Joining forums that relate to your apps and replying threads where you can list the benefits of your app,for example if its a WP Plugin then the WP discussion board will just be alright.
Sites like scribd, esnips, download.com etc will accept ebooks that relate to every niche.So write a free ebook and upload to these sites for users to download and see your app.
Creating feeds and submitting it to feed directories can be of help.
Writing short reviews on sites like ciao,squidoo and hubpages can bring in more exposure.
Write updates and notes on it on your FB profile wall for your friends to see.
Write short articles and submit to directories on the benefits of your apps.
Create a profile on getacoder and leave a link of your site on it.
Go to techcrunch and see if you'll be allowed to write a guest blog.
P2P and torrent sites like rapidshare,piratesbay,h33t etc could accept your app on their directory.