When does a significant p-value indicate a true effect?

Understanding the Positive Predictive Value (PPV) of a p-value

Across all investigated hypotheses: What % of them is actually true?

What is your Type I error (α; typically 5%)?

Do you want to specify power directly or indirectly by specifying sample size per group and effect size? (Assuming a two-group t-test)

% of studies that report a significant result, although it's not

This blog post gives an introduction to the app.
This app is based on Ioannidis, J. P. A. (2005). Why most published research findings are false. PLoS Medicine, 2(8), e124. http://doi.org/10.1371/journal.pmed.0020124
Thanks to Nat Goodman for adding the option to specify power via effect size and sample size.