One of Dave's stated investment principles is to invest (at a lower valuation) BEFORE product-market fit or traction.
I think it's normal in incubator/accelerator programs for some startups not to launch their product publicly before demo day.
Admission to the accelerator program is based on strong recommendations from the 500 Startups mentor network, and then a meeting with the 500 Startups core team.
BugHerd - site down update:js embed widget for website/design changes/feedback
Cardinal Blue - photo/chat collaboration
Culture Kitchen - cooking classes targeting lower-income immigrant women
DailyAisle - online wedding planning
HelloWorld - auto-generated Github resume
Kibin - online editing marketplace (essays, resumes, etc)
LaunchBit - interactive ebook to launch a minimum viable idea (for non-programmers)
LaunchRock - Viral Launching soon widget
Ovia - online video interviewing
Scoopola - 1/2 price last minute concert/theatre deals
Snapette - photo sharing for fashionistas (bags/shoes)
StoryTree - capture & preserve family memories online
Tout - email templates
Vayable - deals and guides to experiences around the globe
VidCaster - Video websites
Volta - A/B testing for outbound phone calls
VVall - Photo capture with memory analytics?
Welcu - online event planning
Zerply - LinkedIn 2.0