There are a lot of good side projects that band members often form for one or two albums (or sometimes more). What are your favorites?

Being a blink-182 fan, I really liked a few of their side projects:
Box Car Racer (2002) - Tom & Travis
Angels & Airwaves - Tom
+44 - Mark & Travis

Team Sleep (2005) was also a pretty cool side project from the Deftones.

On the hip hop side, I'm also a big fan of Linkin Park's Mike Shinoda's work as Fort Minor (2006).

I'm sure I'll think of more.

Gorillaz

Them Crooked Vultures

Them Crooked Vultures is a good one. I also like the side projects from Pearl Jam members such as Brad (Gossard), RNDM (Ament), Tres Mts. (Ament), Eddie Vedder's solo material. I know Mike McCready was in Mad Season but I never heard the album. Matt Cameron also is with Soundgarden again since they reunited. Temple of the Dog which was Chris Cornell and the memebers of Pearl Jam was pretty cool as well.

Robert Pollard (lead singer of Guided by Voices) made 5 great albums with Boston Spaceships

Puscifer and A Perfect Circle

I actually liked Postal Service more than Death Cab

• Mt. Desolation - Side Project from Tim Rice-Oxley & Jesse Quin of Keane
• The Little Willies - Side Project from Norah Jones
• The Postal Service - Side Project from Benjamin Gibbard of Death Cab For Cutie
• The Swell Season - Side Project from Glen Hansard of The Frames

Even though The Traveling Wilburys is more of a "supergroup," I think it could probably be classified as a side project.

Hot Tuna

Anywhere.

When it comes to Green Day side projects' rankings amongst all-time great bands:

Foxboro Hot Tubs > The Network

What about Derek and the Dominos? For Allman at least it would have been side project.
Don't know the particulars. Was New Riders of Purple Sage side project for Jerry Garcia? Didn't Garcia have another band on the side also?

Mad Season:


Sully Erna:




Sully Erna's album is probably one of my favorite albums of the last decade. I can't get enough of it. Hopefully Godsmack fans didn't give him too much shit for it so he'll do another.

Everything Chris Thile has done if we count it as a side project of Nickle Creek.

Forgot about Foxboro Hot Tubs. That album by itself is better than any of Green Day's output from the past ten years in my opinion.

I didn't know anything about them when I saw them open for GWAR, I liked the theme/storyline/stage show and the music, then I found out it was basically the same guys and was even more impressed.

Allen Jourgensen (sic) did about twenty good side projects back in the late eighties / early ninties:
RevCo / Revolting Cocks
Pailhead
Lard
Pigface
...etc. All released some good music.

Golden Palominos has been a consistently excellent revolving cast supergroup / serial side project.

BlackStar was the best thing both Mos Def and Talib Kwali ever did.

Ciccione Youth's one album was great

I really liked Robert Plant /Alison Krauss' "Raising Sand"

Panda Bear's debut was better than anything Animal Collective did before or after.

RD James / Aphex Twin has released music under God knows how many different names and associated side projects, including a lot of his best stuff.

I'll 2nd most of Al Jourgensen's stuff. "Supernaut" is still a awesome cover.

I'll add the KMFDM side project "Excessive Force".

And also Kevin Ogilvie's side project OhGr.

Hindu Love Gods
Zevon & Peter Buck...

Tom Tom Club
Tiny & Chris from TH

Little Village
Hiatt, Nick lowe & Ry Cooder

Rockpile
Edmunds & Nick Lowe

I like his side project stuff ok, but I wish everyday they would get back together and tour.

Also, on the Nickel Creek tip, Sean Watkins with Fiction Family, which also has Jon Foreman. Speaking of Foreman, I love all his solo work away from Switchfoot.

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How To Destroy Angels

Bryan Fallon of The Gaslight Anthem has a side project with their touring guitarist (and now band member) that I absolutely love.