What is the best way to announce a new web app?

What services do people use to announce their new projects aside from Twitter and Facebook? Specifically, if you don't have a popular, established site of your own to announce from. I've used one site (which I won't mention) to make an announcement, but I'm wondering if there are other good ones I don't know about.

I just launched a start-up and we are now in the process of drawing visibility.

A good idea is to be featured on start-up directories. Here's a list:

• Killerstartups
• Crunchbase
• Go2Web2.0
• Vator.tv

Some of these drive good traffic to your website. Killerstartups was particularly good for us. Of course the followers of these platforms are people in the start-up world, and not necessarily your target group. Still, you will get seen, and harvest some useful SEO goodness.

This question on Quora might help.

Spoiler: you need to be a quora member, currently invite only.

I previously got very heavy traffic from programming.reddit.com, but seem to be more picky about shameless plugs than they used to.