I think working alone eventually makes you run out of motivation. If there's one other person who's as active as you are, and you can work on sub-parts at the same which are complementary than your motivation will remain high.
One of my current projects is implementing an OpenId Consumer library (yes, NIH at play but I try to justify it by noting PHP has no true OOP PHP5 implementation just yet - the JanRain library is PHP4, is unable to meet the PEAR standard, has global and procedural function litter, a fair amount of responsibility duplication). It's an interesting challenge with a few sides - the OpenId specs for 1.1 and future 1.2, and its reliance on some heavy handed big integer math like HMAC and DH (Typekey also needs DSA). Trying to implement the Diffie Hellman Key Agreement algorithm is just one of many OpenId challenges - thankfully PEAR has the others

.
Still, the point is, it's fun to implement because its new, challenging, and I have a partner in crime to bounce it off since he'll be roped into adding it to a PHP extension. The feedback helps keep me focused and the project fresh. Working alone, I'd just end up getting bored once the challenge dried up and start wondering whether it should be archived for a rainy day (that never arrives

).