I wrote it in eight days with a budget of $60, and recouped that within three sales. It has been profitable since. I don't know if you would call it a lot of traction -- by Twitter accounting I have something like two hundred thousand users. Anyhow, 4 years and 3,500 odd paying customers later, it was paying my full-time salary, and I quit the day job. That was about 7 months ago. BCC (and consulting) gave me enough time, money, and smarts to get my next project ready.
http://www.appointmentreminder.org
That launches in 48 hours. Depending on one's optimism in counting promised orders prior to the cash physically being in one's hand, it is already profitable.
1. Some companies have the revenue from day one, but never reach profitability
2. Some don't have revenue at day one but are profitable.
For example, most consulting businesses, when started by people who already have the chops for what they're doing and have clients lined up, tends to be like #1.
A company that needs to scale up it's online presence to be effective (for example, something that works on user network effect like ebay/craigslist/etc.) may not take in money during the product development phase, tend be be like #2.
You can also be a hybrid, like 37 signals that went from consulting and design for others to selling their own apps. I tend to think that this is one of the better methods of bootstrapping - you get exposure to customers and business methods, and can see where needs are, then target them, all the while taking in revenue.
The downside is that bootstrap method is that your consulting could cause contention for time/resources - you have to want to do both and be good at balancing them. If this is an issue, it may make sense to look for investors to put in capital to cover the dev phase.
When I started Lighthouse, we were "profitable" right away because of a few strategic deals. Our only monthly bill was to our CC gateway. At the time, my partner and I were both consulting to make ends meet. Essentially, we were working our asses off on two jobs and only getting one real paycheck out of it. I think it was about a year before Lighthouse could sustain us both.
An e-commerce site, I talked with a number of customers before launching (wish I had something more fascinating for you). The site fits under the category of scratching my own itch since I am a scientist. I wouldn't consider the site as having lots of traction nor expected it knowing the customer base. At the moment am focused on customer's problems and am quite dependent on word of mouth.
Bought a template of ThemeForest, customised it, and created a filtering plugin, then launched it. Received 50,000 uniques in the first week through blog posts. Earned around $6000 from it in 3 months.