Hopefully you had a pre-launch page where you collected email addresses, then you can email these people upon launch. Facebook or Google cpc ads. Submit your site to business/product directories. Post about your web app on relevant forums (ie. if it is an app to help accountants, find some accountant forums to post on). Join linkedIn groups and post about your app in there. Create a facebook page/twitter and post upd…
Hopefully you had a pre-launch page where you collected email addresses, then you can email these people upon launch. Typically this is a pre- development page, whereby you're basically tricking people into thinking you already have something, just to gauge the viability of what you're thinking of building. In other words, it's a sleazy practise.
If you are solving someone's problem, they will be happy regardless of your development state.