Ludum Dare has been running since 2002, with tens of thousands of entrants at every level of proficiency. As an industry-wide event, participation carries a sense of being part of something, and there's no shortage of interest or discussion online.
The main Ludum Dare event happens once in a year, but throughout the year, a lot of Mini LDs are held. Since I don't expect any great results at the main LD, which has countless participants including pros, I only do mini LDs. That's a lot of fun, because the community is large and people actually play your game in the events.
The fact that everybody gets to VOTE for an entire week to generate a shortlist and final theme which is announced in real-time to great fanfare is half the fun. It draws you in, gives you ideas beforehand, and generates a conversation online in the days leading up to each wonderful #LD48.
It is like a drum roll that gets you excited.
Multiple developer have set up streams allowing people to follow development process in real time. There's a public directory on Twitch with Ludum Dare streams.
The goal of #1GAM is to release one game every month. The challenge is about maintaining that frequency, so there are no restrictions on any single creation. It doesn't matter when the game was started, how it was made, or even whether it's a computer game: "AAA" industry titles, games made during other jams, clones and ports of existing titles, and physical board and card games are listed among qualifying examples.
Many jams follow a regular schedule, but the actual event is only a few days long. In contrast, #1GAM is continuous and ongoing. Submissions are always accepted, and count toward the current calendar month.
For extra encouragement, One Game A Month is a game! Participants are given experience points and publicly-visible achievement awards for individual releases, and for keeping an unbroken release schedule. Leveling up and stocking the virtual trophy case can become additional enticements to stick with the challenge.
Based on the principle that learning happens faster in groups, teams are strongly encouraged. In particular, the BaconGameJam community favors impromptu teams formed on their reddit community before each jam.
The rules and FAQ make reference to solo jams where teams are not permitted, but every BaconGameJam so far has allowed teams of unlimited size.
BaconGameJam aims to be all-inclusive, encouraging participation regardless of skill. Those who can't code at all are invited join a team and contribute in some other way. Code-free game creation tools are also explicitly permitted.
The single-weekend format of most jams can be daunting to the beginner, but on the other hand a short deadline is a great motivator. InsanityJam allows two full weekends for development, plus the week between them.
The theme of InsanityJam is that every game is based on an individually-generated random idea. This takes focus off whether or not the idea is any good, bypassing a lot of potential hesitation. It can also help get participants out of their creative comfort zones.