I am new at this, but I will part with what I know. There are thousands of festivals all around the world. So where to start?
Source 1. Chris Gore's Ultimate Film Festival Survival Guide
If your going to make a film buy this Before you start shooting, it is well worth the money and I wish I had read it prior to filming. And it is really fun to read.

There are three parts to this book.
The first part is general festival knowledge, what to do and not do.
The second part is director interviews, at first I ignored this section but you can learn a LOT about films that did really well and films that tanked in this section.
The third part (half the book) is film festivals, it has one major list then a ton of awesome sub lists.
*funny story* I've had this book for several months and I'm sitting there (on the can) and i see a Dances With Films advertisement. I'm like "damn that picture looks just like a shot i did once".... Now the picture is in black and white and not color so i wasn't really sure, after a few moments i see a tiny print "courtesy of Ghosts of Hamilton Street" which is a movie i shot a few years ago. I almost flipped out!!! How cool, a book I'm learning from has a full page ad of a movie i shot. SWEET!*
Source 2. Withoutabox
It has listings for Thousands of film festivals and a lot of tools to make your like a lot easier.

This site is amazing, first off you can setup a film project and search for festivals that qualify for your project (this is free). Then you can submit to festivals online (this is free). THEN you can buy a prepaid discount that saves you money each time you apply to a festival (not "free" but will save you money if you enter a lot of festivals). Just sign up, the message boards alone are worth it, this is really the best free thing out there for film makers.
Thank you Mike Miller for showing me this!
Source 2. Film Festival Secrets

Chris Holland has his hand on the pulse of film festivals, download his FREE book. Let me repeat that, FREE. Also he offer a film consultation, here is the short film break down:
The $99 Shorts Special (he also does features)
For short films (under 40 minutes) only! Filmmaker receives a written report on his/her film with the following:
* I (Chirs) watch the film and comment on my opinion of its overall festival viability.
* Submission tips and sample cover letter to an upcoming festival of your choice.
* Completed festival pyramid (prioritized festival recommendations as described in my book) with recommended festivals for your film's specific genre and content.
* Suggestions for branding/giveaways/merchandising to do at the festival
In addition, filmmaker receives one hour of phone consultation time. This is a great deal for filmmakers looking to make the most of their short films on the festival circuit.