Being so new to fandom, I haven't been in a LOT of challenges/communities/etc, but I'm excited about the ones I have done. I'm coming at this from the angle of being primarily a (new-ish) podficcer.
ITPE: The Informal Twitter Podfic Exchange! Signups happen on twitter, of course, and the challenge runs from Oct 15 to Dec 17. It's "secret santa" style, and the podfics are sent to their recipients on Dec 24.
I signed up for ITPE on a whim, pretty much. I wanted to stretch my wee podficcer wings, so to speak, by having to record something I would possibly not otherwise have recorded. It worked-- I ended up getting someone as my partner who has vastly different taste than I do. I struggled for a while with what to record, but ended up going with my favorite canon-era Merlin/Arthur fic in the end. I could probably have done better catering to my partner's taste, rather than to my own, but hopefully they will enjoy it when they get a chance to listen to it! I had a lot of fun, anyway, and it gave me some good experience with podficcing explicit fic and forced me to be more careful with my audio quality (I had to re-record and edit the entire thing because my old headset mic was awful). Plus, it was my first challenge and my first podfic with a deadline. I liked the informal nature of it quite a bit, too.
Fandom Aid: I found out about Fandom Aid through some friends of mine tweeting about their offerings for the Typhoon Haiyan fundraiser. It's a Livejournal community where "like minded authors representing hundreds of fandoms, and countries of the world, have come together to offer custom-written fic in support of those affected by natural disasters", in their words, only they also accept offerings of fanart, podfic, fancrafts, and anything else fannish that can be donated to support relief organizations.
Why I love this should be obvious, but to elaborate, I'm not exactly wealthy, so any time I can DO something for charity rather than donating money, I'm all in. I offered three podfics of any length and fandom, and ended up "selling" two of them. This was even more of a challenge for me than ITPE, because one of the fics I ended up recording was not the sort of thing I would normally read and was emotionally difficult for me. I learned a lot about my own boundaries, but I also made some pretty decent podfic, and all in the name of helping those affected by a disaster. As someone who lost everything in an apartment building fire at age 15 and was helped enormously by the Red Cross, this means a lot to me.
Amplificathon: You might think anyone who listens to or makes podfic already knows amplificathon, but since I was so recently new I'm well aware that not everyone comes pre-acquainted with all the available resources, so. Amplificathon. A DW community where new podfic is posted, and anything posted there gets archived into the Jinjurly audio archive.
Not only has Amplificathon given me a place to easily find new podic, but it's given my podfics exposure (and, of course, the benefits of being archived). It's pretty much The Place for podfic, as far as I'm concerned, on DW.