Which projects are the most fun/useful and how did you decide to get started with them?

Most fun in the real meaning of this word was Belgian Beer, and is Pirates. But both was never done a a whole time job, and never intended to be one.
I came there through the community, joined, crunched, posted and sometimes laughed.

Most useful are those projects with a good and straight path for the results to get in the open, and with quite tangible results in the form of either scientific papers or reached goals, and that do something that's relevant for mankind, i.e. not looking for primes.

I used to get started with nearly every new project that popped up somewhere in BOINC, exemption only obvious commercial or probable malicios hacker projects. I now pre-select a bit more, I won't join new prime projects (and probably not other maths only projects without connection to the real world), I still won't join most commercial projects (perhaps for testing, if they look quite serious), and I won't join those rainbow-table e.a. malicious hacker projects that pop up from time to time.

I've got one project, that I run next to always: CPDN. It's the second I joined at all (after Seti, which I don't), it's got a very good community and responsive admins and mods, it's a very good science, it can keep my puter busy even for longer off-line periods and it has one sub-project that suites my OS/CPU-setup very fine and thus grants quite a lot credits.

WCG has one of the best evaluation processes even before the start of a new sub-project, so they can be definitely defined as "useful".
Simap puts all it's result in free-to-use tables (or whatever) for scientists to use them, and they seem to make good and wanted work.
Some of the protein folding projects take part in the scientific test CASP and have usually good results, another proof of their worthyness.

Sort:
To start for me there's not much more needed than a good story and a believable webpage.
To keep me, the project must be responsive, grant fair credits, do useful science.

here are the projects I'm currently crunching, and sorta why... Some projects I do because I believe they're actually doing some good, others just for fun...

--BURP (Big and Ugly Rendering Project): Putting together a BOINC-based rendering farm, using the Blender 3D modeling/rendering software. Currently allows rendering for free, although at some point you will have to contribute some CPU time before you can have something rendered. Current licensing allows non-commercial rendering only. An Alpha-stage project, that is closed to new registrations of the time being.
--Engima@Home: Help decode Engima-ciphered transmissions from WW2, that are still unbroken.
--LHC@Home: Help improve the operation & efficiency of the Large Hadron Collider at CERN.
--World Community Grid: Currently only Clean Energy and Nutritious Rice sub projects.
--Spinhenge@Home: Something to do with nanotechnology.

Projects I'm not currently rendering, but might again:

--CPDN: Climate models; pretty long WUs, can take as many as a few months for one to finish. I stopped because it starves other projects, although multicore computers don't have this problem as much.
--SIMAP: Updating publicly available bio-informatics database.
--Rosetta@Home: Cancer research at University of Washington
--Einstein@Home: Gravitational wave detection

I'm always trying to improve my mix, so that I feel like my FLOPs and kWh are doing something useful... I won't crunch anything without a clear description of what the project is doing, and what its progress is. I would like to trim my projects more towards sustainability, but anything cool or useful is game. Currently looking at Hydrogen, APS and IberCivis as potential future projects. I also have it trimmed to 5 projects, so I'm not "throwing my eggs" all over the place.

I usually go for the bio-science projects. I started with SETI, like most people, but they now have only half a time-slice. Otherwise, my little farm cranks out a lot of work for WCG. Lately, my triple-core PC seems to be Docking / POEM / WCG only. I also pull a lot of SIMAP work when it's available. I crunch for Einstein, because I think the work is important (both the high resolution stellar mapping, and the proof of gravity waves). When I can get Orbit work, I crunch that. (Still waiting on a ~300 hour WU to validate.) I know the project is still rather beta-ish. I think curing cancer would be a moot point if something hits Earth and incinerates the surface.

I also run Folding @ Home, which is non-BOINC. Again, the medical value draws me. (I've lost friends and family to some of the diseases they study.) I have the CPU client running on two computer cores (an AMD Athlon 64 core and PPC G5 core), the GPU client running on an Nvidia card, and the PS3 client. Actually, the PS3 stays on, just like my computers, and has cleared 1000 work units.

I run mainly medical projects because of cancer in my family.

Cells - currently down and changing locations. Tests why cancer cells break free and spread throughout the body.
Docking - looking for new types of drugs.
Poem - one of the docking programs which participates in CASP.
SIMAP - public database of proteins. Usually only has work the 1st week of the month.
Superlink - gene study.
World Community Grid - has multiple sub-projects including one for cancer.

I also run Yoyo on my old G4 Mac to make sure it always has work.

I am computing towards the singularity. That means projects that could result in a better hardware/software performance first, then projects that deal with the mind or AI's, then biological projects, then nano-releated projects, then chemistry/physics.

Of course, this means pretty much all the projects

I don't like the "numbers" projects because, while they may have a practical utility someday, my resources go to the ones that have a better chance to improve our lives.
Same goes for SETI: while i hope someday we'll discover and talk to aliens, it's an "all or nothing" project, and if we don't "hear" anything, we don't get anything from that project. Not saying it shouldn't be done, just sayin' there are more worthwile projects out there, and too many FLOPS go to SETI compared to it's practicality.

I crunch SIMAP for the science and the scrambles. . .

. . . I crunch PrimeGrid for their excellent Challenge Series. . .

. . . I crunch Rosetta for the science and a change of pace. . .

BUT my home project is World Community Grid, especially Help Conquer Cancer and Help Fight Childhood Cancer.


My Mother and Sister died from cancer and my wife is a breast cancer survivor.

. . . from personal experience, I have seen the tremendous progress in cancer treatment and research in the past 30 years.

I have taken the women in my life to many chemo and radiation appointments.

. . . and I seen entirely too many children and their anguished parents fighting this ugly disease.

I crunch, hoping that I might contribute a bit to making a world with fewer bald-headed children.

I'm currently crunching the hell out of WCG / Finding Dengue Drugs, because I live in the most affected country. Even though TV is ignoring the 10k infected people and counting the swine flu deaths instead. Maybe when our winter ends and mosquitoes come back, they'll remember dengue existed...

Also crunching only just enough SETI WUs to have enough RAC to post on the forum

After crunching for a while, one gets down into a groove with their favorite projects.

I look at WCG, SIMAP, Rosetta, Einstein and PrimeGrid and see

. . . the rest, more like